Is that true? Mechanically it's true, but I don't know whether the game code actually knows this is true. It depends whether the N64, and the SM64 code specifically, tracks the button state and the events separately.

Compare the situation with Left + Right in TAS Mario 1. The TAS has no problem telling a console that both left and right are pressed, but obviously a real human player cannot press both directions, they're opposites. Nevertheless this does "work" in the game and so although no-right-plus-left is an interesting benchmark to compare against RTA where live human players couldn't do the trick, the TAS records allow both pressed at once.