Basically, coherent in Forge jargon means that the game has rules for what it's thematically about, and nothing else. Consider a game like My Life With Master: it's a game about henchmen of an evil villain who find love, and this love may or may not give them strength to overthrow their slavemaster. This story is the only one it is designed to tell and trying to do anything else with it requires the group to actively fight the system. By contrast, any sort of traditional sandbox game is "incoherent" because it allows the group to pick and choose what to do with it and the game doesn't try to actively constrain the options of the participants towards a pre-determined theme and mode of play.It's been a while since I read the Forge stuff, so I'm not sure what "coherence" means in their loaded jargon. I wouldn't say that content-oriented rules need to be incoherent in the normal sense of that word. They can easily drift off into rambling incoherence, since there is no actual pattern for the author to follow, no ultimate way of checking if a given rule is where it needs to be.
I do note that the story games of today are not as hyper-specialised as Forge games tended to be, but they do still have a much more guided style of play - consider how Apocalypse Engine games has a set selection of moves that players can take that have been pretty tightly scripted into the system and to go outside them is to implicitly break the game, certainly house-ruling at the least. By contrast, something like Godbound gives you character creation rules, combat rules and some sub-systems (and a lot of random generation tables for improvisational support) then gets out of the way and allows - and indeed, expects - the participants of the game to make their own direction.