I believe Sing is buffed to some extend!
did some counting on the frames, it actually looks like the Sing hitbox will be active on Frame 16! (thats the frame when the music notes appears)
Frame 16 is Huge! especially when compared to Smash 4s of frame 29!
if its Frame 16, its as fast as Marios FSmash in Smash4!
but since no one was put to sleep on the first hitbox, this is all speculation for now
did some more counting and it looks like the whole animation is 160 frames which is 20 frames faster than Smash4 FAF.
speaking of FAF, it looks like the FAF in Ultimate is gonna be 130 frames. this is speculation since the jigglypuff didnt move at all after sing and a short while after.
another buff I think is potential is that the hitbox will be constant active when used.
since 64, the Sing had 3 hitboxes that were active and in sync with the effect: when the effect were visible, so were the hitbox.
in ultimate however, the singing effect were always visible! in theory, the hitbox should always be active!
while its not the buff everyone wants, a buff is still a buff