It isn’t true?
I’m not aware of anything that say it isn’t true…I’ve always been taught it was and read likewise…also makes sense to me when you think of the size principle etc….
Well all of those squat jumps, dynamic/explosive squats, medball throws, and the like must be recruiting tons of slow twitch fibers because they are way, way below 80% of a 1RM for the given movement and joint angles. Enoka has research that indicates there are not significant differences in muscle recruitment >80% of 1RM and the differences come from rate coding and other factors, not muscle recruitment. If you are moving the bar with the intent to move as fast as possible, it is going to make this discussion almost completely irrelevant.
I mean, you have to even question what he says in the article you are referring to from AthleticsCoaching.ca. Somehow, doing 4×4 @ 80% is recruiting IIA and not fatiguing or hypertrophying the IIB fibers, but 5×3 @ 90% (how is that even possible to do?) is magically recruiting IIB and training them better?