ACKS is a splat book fest so it's disqualified. Not that it's a bad game.
ACKS core is its own game that you can play with just the core book. If you can play a game with only it's core book than that one book is a single book game that does not require other books, whether or not there are splatbooks available for it.
My thread, my rules. It doesn't matter if you can play the game with just the core book; if there are tons of splat books full of optional rules, extra classes, mass combat, it doesn't count. Adventure modules are fine.
DCC could count since in my estimation the annuals (which aren't annuals) hardly rise to the level of splat, nor do fanmade things count. Though the various boxed sets with alternate rules and classes make it questionable. Technically I guess they're modules.
But ACKS? No way. In its own thread they note how they're struggling to put everything together in just three books for a second edition. All those theme books, extra spells, classes, etc. It's creeping towards AD&D 2e territory there
OSE would count, but OSE Advanced would not, since it's two books. Hyperborea 2e counts, and is way up there for me, but sadly Jeff released 3e in essentially 3 books since there's also the gazetteer.