I could see how this could be confusing. SM64 does distinguish between before a release and after a release, so technically the game itself doesn't care. I think what they were trying to say is that there really isn't any benefit in speedrunning the game to keep track of releasing (and this can be confirmed in the video too: https://www.youtube.com/watch?v=kpk2tdsPh0A&t=105).
I think it's possible that if the next level requires .5 a presses but, for example, you are falling with a wing cap in the current level, you have to decide between bumping that half a press into a full a press or losing time when falling with the wing cap. I just don't think it's a common case though because you could use a different route where the held A won't affect you. It looks like in the A Button Challenge (ABC), there are only 5 half a presses: https://ukikipedia.net/wiki/A_Button_Challenge
I think it's possible that if the next level requires .5 a presses but, for example, you are falling with a wing cap in the current level, you have to decide between bumping that half a press into a full a press or losing time when falling with the wing cap. I just don't think it's a common case though because you could use a different route where the held A won't affect you. It looks like in the A Button Challenge (ABC), there are only 5 half a presses: https://ukikipedia.net/wiki/A_Button_Challenge