This is totally baseless. Susanoo's arrows don't leave time for handseals, unlike the Bijuu Dama.
Yes they do, Susanoo arrows have to be preped just as that TBB did.
Don't you see Sasuke aiming, or forming the bow and arrow?
Sasuke also has to form a Susanoo before he can uses Susanoo arrow, all this adds up to prep for using the arrow.
Sasuke can insta spam Susanoo, and release SA so fast that Kakashi didn't even realise a projectile had been released until it was half way through the air. The Bijuu Dama's required prep time is noticeably longer, and Minato had time to use hand seals.
Tailed Beast ball is much much slower......
its like comparing which one is faster, a hand thrown football or a arrow...........obviously arrow is much faster because of streamlined shape and force produced by the bow to launch it.........
We saw Sasuke prep the arrow, we even saw him aiming there was nothing instant about it.
We have never seen using Sasuke Susanoo arrow with a partially form susanoo , he always fully forms Susanoo when he uses the arrow.
For dramatic effect, obviously. Futher, that was MS Sasuke, EMS Sasuke seems to be able to almost insta spam stage 4 Susanoo. But yeah, Bijuu Dama's have longer prep time.