Note that in theory up there? The first time I and a couple of friends tried playing third edition, I was DMing. By that point, all of us had a couple of decades of tabletop experience under our belts each. As such, after a few introductory adventures, I found that their pairing of an elf wizard with thief levels added in and a halfling barbarian were consistently able to take down challenges suited for twice as many players and at least 3-4 levels above their own. I for one just chalked that up to experience making all the difference...
Note, the rule of thumb I listed was for creating encounters for low level adventures, aka levels 1 through 3. By level 4 I'd start throwing more things with 2 hit dice or more at the party, or with more exotic abilities, or a combination of such features. Basically, starting with level 3 I'll track what the party is actually capable of, and design adventures to specifically challenge the party's actual capabilities. A party with no thief in it, I'll probably have encounter fewer traps. and that's one example.
Also keep in mind that Power Gamers have
always thrown off the power curve of any given RPG. Some people just aren't happy unless they've made the most broken character possible that trivializes and defeats every encounter (including adventure boss)
before the first initiative round. It's how things like the infamous "Spiked Chain Fighter" build for 3.5 came about. Said power gaming was... less possible in 2nd edition, because you flat out had fewer options. But 3rd edition and 3.5 really brought power gaming into the limelight. In truth, the gaming group I was in when 3rd edition first came out looked over the players guide, ran a few adventures with 3rd edition, then went right back to 2nd edition AD&D
because it made player characters too powerful too fast.
Yes, in theory the party's enemies also have access to those same easily broken feats. But that's a lot of things for the GM to keep track of. In truth, my biggest problem with Pathfinder Society Organized Play was the fact that as a GM you
had to allow those broken characters, so long as none of the feats or equipment are specifically banned by PFS Organized Play and the player can prove they have ether physical copies of every book used or a watermarked PDF of the books. Even worse, as a GM you couldn't disallow things you don't have personal access to, which can be problematic if someone's running a class or feat combo you don't have the rules on.
I took the time to print out the entire section on the Kineticist class from the book that's in, as well as the pages with every ability and feat I was running. That way I could hand any GM the rules for my character if they weren't familiar with it. But I was definitely in the minority for doing that.