Absent anima powers, two characters that hold Melee as favored that spend the same amount of their earned XP on melee charms will be about equal in skill when it comes to stabbing people. If the Zenith is just as good with Martial Arts as the Dawn, and the Twilight is just as skilled at Archery as the Dawn, and the Night is just as good at Thrown as the Dawn and the Eclipse is just as good at Melee as the Dawn, then the Dawn player is sort of lacking in things that make them unique.
You could always take a page out of the Lunar storytelling chapters though, differing characters by having them use different weapons. That kind of advice always went over well with the fandom in the past.
Actually, there are quite a number of other skills that aid in combat. That's one of the problems in the 2ed system, where Dawns would have a bunch of redundant combat abilities while also needing to pay quite a bit to cover some necessary other aspects(Awareness, defensive charms, etc). Now, this has been addressed in 2ed and in 3rd, but not by removing these things. Simply by making Dawns more able to chose this abilities without any taxes. So, yes, two characters who invest equal resources into these abilities will supposedly have equal capabilities(ignoring suboptimal charms and the like). The cost, however, is that this means the characters can't spend resources elsewhere: If I make a Dawn Combat/Socialite who spends 80% of his xp on combat or combat related areas he's going to be about as good as a Zeinth Combat/Socialite who did the same, but neither are going to be as good at Social stuff as a Zenith Combat/Socialite who went 20%/80%.
Your comparison also breaks down because comparing two characters in the areas they are similar while ignoring the areas they differ kinda misses a big issue. Yes, if you take two characters and have them spend XP the exact same way then they're likely going to be very similar mechanically and in how they play. But having two people with melee doesn't mean that you have that. Not only is melee big enough for variety to exist within it (a light weapon user will feel different than someone wielding a massive hammer), not only can you have different degrees of competence (one spends 50% of xp on combat stuff, the other spends 20%), but Melee is one of 10 favored or caste abilities.
Oh, and lol "absent things which make one person better than another at this skill, both are equal". I mean, yes, that's valid, but the premise makes the argument absurd.
And sure, the greatest sanxian player in the East has Charisma 5 and Performance 5 and a (Sanxian +3) specialty. But that doesn't change the fact that everyone is pretty much expected to start with Dexterity 5, Combat Ability 5 (Specialty +3) or at a similarly high level of proficiency, with a serious charm investment to back up the dots. Requiring people to shoulder the burden of beginning play as "their actual concept + Dawn level skill in one or more aspects of combat" is a flaw in the system.
I've played in enough games where everyone. regardless of their actual character concept, happens to be as agile and beautiful and willful and as skilled at combat as a human has the potential to be, simply because Dex 5, App 5, Combat Ability 5, Willpower 10 are cheap buys at chargen. Righteous Preacher? Peerless Kung Fu. Librarian Sorcerer? Well they also happen to be a swordmaster! I just want to play a Cunning Courtier, not one who is also an Assassin, but when everyone is kitted out for combat I have to be kitted out as well.
There are two things here. First, a non-insignificant part of that assumption is the XP/BP divide. If you change that, it becomes much less difficult to have non-5's in core areas. Dex and willpower still have weight behind them due to being god stats (Dex is king in combat and many out of combat applications, willpower gives you motes, mental defenses, mental health bars, charm costs, etc). Especially if you go to a full xp system, where getting that 5 in an attribute or ability costs a lot.
Second, it's pretty easy to not do combat at that level. How much depends in your ST, but even for pretty hardline "I'm going to run the world as full of extremely competent people" ST's there's plenty for players who aren't at max mundane skill level to face, and at most it means that certain high level fights need to be avoided or set up in such a way that it's not a white room fight. I mean, remember, non-solars exist in the game, and they have lower dice limits. The fact that they can overcome challenges means that the Maximum solar dice cap isn't strictly necessary. Hell, mortals are a thing, and they too can overcome stuff. It makes combat easier, especially against weaker foes, but that doesn't need to be a given.
This is especially true if one person on the team is combat focused. Yes, they're combat focused and will be able to fight things evenly that you can't. That doesn't mean that suddenly you need to get to their level. That just means that you won't be fighting the same things that they are evenly.
Yes, a lot of people will go 5+5+3 with large combat suites, but that's because a lot of people find combat, or the idea of combat, fun, and thus want to be able to do a lot in combat.
An actual samurai would be skilled in Melee and Archery. Achilles invested in Melee and Thrown. I can continue, but instead I will just acknowledge that, yes, this can be a problem, because despite real-life examples almost always have two ways they used weapons we tend to just invest in one, but it's part of why the Martial keyword was created.
I know I don't have to tell you this, but to make a point: don't forget War, Athletics, Awareness, Dodge, Ride, and Resistance at minimum.
Combat skills are important for combat, yes. But if you had a character who was 100% in Melee and a character who also had a mix of other combat related skills, well, I could easily see the more rounded character doing significantly better (hell, in 2ed the 2/7 filter relies on some of those other abilities).