If I'm not mistaken, muscle fibers can only fire maximally--a muscle fiber either fires (maximally) or it doesn't fire. Isn't muscle fiber recruitment what differentiates delicate, precise motions from gross, explosive motions?
As you train (that is, try really hard to lift something heavy), you teach your brain that you're going to be lifting heavy things a lot going forward, which makes your brain recruit more muscle fibers when you actually go ahead and lift the thing.
So you're right in one sense, and incomplete in another.
Also why wouldn't evolution just make us all strong all the time if it only requires neurological changes?
For very costly things like strength and brainpower, evolution is ensuring that we have as much as we require, and not a single bit more; and has all kinds of processes for actively reducing strength that you had unless you have both extra calories and the demonstrated need to use that strength; because it saves calories.
It's one of the theories of why homo sapiens out competed neanderthal basically everywhere, we can survive the lean times better.
Iirc they required about twice the average intake of an anatomically modern homo sapiens.