Parenthetically, I'm going to guess that it would take an exceedingly long time for an infantry unit to be even half as good as a natural-popped archer.
So long as they have nothing better to do, which is pretty unlikely I suppose, they might as well. Even with the limited success of an infantryman's archery it might prove useful in special cases like if the enemy simply cannot be hit outside of a ranged attack.
But if other units had training after popping, they could also learn to rule.
The main question is whether intelligence is dependent on unit type or is based as a unique trait for individual characters.
Bogroll was stupid but are all twolls dumb? Even if they are was Bogroll exceptionally stupid? Mildly stupid? Or actually smart by twoll standards? We haven't met any other twolls so it's hard to say.
I'd say casters in general are a fair bet to be quite intelligent considering the conversations that Parson has had with them but then again we've had few examples and only Sizemore had much a sizable amount of knowledge outside of his discipline. In fact I'd wager that Stanley's incompetence can be mostly explained by the lack of knowledge outside of his original unit type, Piker (if he even changed types at all).
King Saline most likely taught him some things but I doubt that he trained him enough by the time he was killed. And Stanley doesn't strike me as the best of students, he seems to need to see firsthand the practical value of something before he agrees to learn it properly such as Parson's Intelligence.
Not to mention that Stanley, Wanda, Sizemore, and Maggie seem geared (at least initially) to see a failed attempt as a defeat rather than something to learn from for the future. Ansom seemed geared that way too until he was decrypted (though that might be more due to being "Popped again").
Heh, come to think of it, Parson "popped" (or PLOT-ted) with the general knowledge on how to break games and in TBFGK that's essentially what he was trying to do in order to win, rather than play by the rules as they were intended he sought out exceptions and side manipulations, he was just two steps away from outright cheating, no wonder Stanley didn't like how Parson won the battle.
I could just imagine players around a game board:
"Oh come on! That's not fair!"
"It's not against the rules."
"But it's cheap!"
Except when you've killed hundreds or thousands of people, then it's a little bit different.
Edit: Note that by "intelligence" I mean nothing more than "capacity to learn".