You're kinda right, but I feel like you're still wrong.
A MDV of 4 is a pool of Will+Integrity+Ess of 8, a Heroic mortal with no investment has a pool of 6, one with an Investment of 1 in Willpower and 1 in Integrity, they hit a pool of 8, a fairly small investment, especially as Willpower is a valueable resource is a wide number of situation and is super-cheap to increase during character creation.
That last bit further is complicated by Exalted character creation, Exalted want Will even more than mortals and the character creation rules would seem to encourage maxing it then. I see a MDV of 10 and it reads as "A chargen Solar with 3 Integrity and a Integrity Excellency can beat that. a Chargen DB with 4 Integrity and an Excellency can beat it". Like it's probably more impressive than it seems but if you've played the system a lot it just seems....low.
A heroic Mortal with no investment as an MDV of 3, 5wp + 1 Essence is 6 and halved is 3.
The heroic Mortal in the antagonist section of Core has an MDV of 6. (Essence was errated down to 1)
A Chargen Solar can do rather a lot of things, if they invest in it. Pretending they got WP 10, Integrity 5, and Integrity Specialty against Lunars 3, and raised their essence to 3 their MDV is 11, still a point under what they need to spot a random Lunar if they aren't specifically looking for one. So even a fairly optimized Solar must either be in paranoia (in the none combat sense) or use magic to see through the zero investment defense of the tell.
You know who doesn't have MDV 12? Literally, everyone that isn't a hyper-optimized PC, godlike Elder, or significant spirit. Like all those non-heroic mortals guarding the grain caravan who won't notice that their drunk comrades who went to throw up now has backward hands or what not. If you are curious about who else doesn't have MDV 12, the list includes storm mothers, Gri-Fel, and Fakharu - Censor of the West!
Who does have a high enough MDV to see through the tell without magic? Celestial Lions, most Third Circle Demons, and various extremely high ranking gods.
PCs are not built like "regular" people. They are usually hyperspecialist zealots whose entire life apparently consisted of nothing but training to accomplish the exact thing they are meant to be good at.
But, conveniently, Lunars can add other magic to make themselves more stealthy, which they really should be expected to if they are going against beings potent enough to see through the zero investment concealment of the tell.