I'm 100% positive that you don't need to double tap the analog stick to BDACUS.
Or you do, but when I singled tapped the control stick it resulted in sliding usmashes that just happen to be completely identical to BDACUSes but aren't BDACUSes. (Yes, this is sarcasm. They're BDACUSes, lol)
I'm gonna go ahead and believe what SFP says because
-Again, I've been taking advantage of air release followups with ZSS that all start with buffered dashes since I joined the scene competitively...which was over a year ago.
-He was just given frame advance and was testing out tons of **** recently, and given his ability to provide the frame data for buffering dashes out of ZSS' air release on other characters, it seems like he probably tested it
-His explanation for why people thought it was necessary to double tap makes perfect sense
Edit: FTR, I've BDACUSed with Sheik, Falco and Diddy before. I can't get the full length out of regular DACUSes for any of them, so if I'm ever playing them for fun I BDACUS rather than trying to do a normal DACUS. You only need to tap the analog once.
Buffering dashes backwards is probably the same, but the difference with that is if you try to buffer a backwards dash + something else, you'll turn around instead.
eg by tapping forward on analog --> pressing down on C-stick in the buffer window, you'll buffer a dash attack forward
However, if you try to do that backwards, you'll buffer a turnaround dsmash.
So to buffer a backwards dash + something else, instead of pressing forward --> w/e, you have to press back --> down --> back --> w/e. Like in
this video.
eg in the buffered standing pivot grab infinite DDD has on Yoshi, if you're facing right, you'd have to buffer
Left --> down --> left (all on analog) --> right on C-stick --> grab.
And I'm sure you can see why this infinite barely affects the MU. It's unrealistic to expect someone to do this consistently.