I'm not convinced that they have SPD 'loaded'. They've been playing a long time, and are likely doing SPD on reaction. In the earlier games, ST, alpha, 3S, there was a lot less input leniency, so to do 360's from down back you had to do the motion really quickly, so that the character was still in crouch animation as you were doing the up directions. These players have liked built up the muscle memory to the point where they can pull it off instantly.
That's not to say that buffering into f.mk, dashes and empty jumps isn't happening, but I think trying to buffer SPD's in neutral is only going to cause problems for you, especially when moves actually hit and you try to combo, but the buffered SPD comes out and you end up eating a full punish. It's best to buffer SPD's into jabs/shorts close up for tick throws, and if you see a move hit, press either s.lk or c.lk instead of punch to combo.