Secondly, because using up-B in hitstun-escaped knockback also gives you the huge boost up and away from the stage I just tested.
Well, whaddya know. KID wins, I got a couple things wrong.
1. When you're in hitstun-escaped knockback (everyone alright with that term?), using even a jump, or
any of Sonic's specials, gets you an unfortunate magical boost up and away from the stage. Nevertheless, you probably want to be doing any of a mid-air jump, fast-as-possible side-B, or fast-as-possible down-B to avoid the side; the momentum they each regain you is worth the momentum penalty you get for using them in knockback.
2. After extensive testing, it seems that each of those options (jump, side-B, down-B) are equivalent in their ability to save you from the side. So use whichever you like, if you're comfortable pulling off a side-B or down-B as fast as possible.
My experiment is the following: training mode, Final Destination, place DK at the farthest left of the stage, and put Sonic in the middle of the tip of the chevron pointed at DK, C-stick a DK f-smash. At 94%, if Sonic uses his Fair asap in hitstun and leans toward the stage, he still dies. ASAP Fair > 2nd jump, quick side-B, or quick down-B means you survive, up to 102%. At 103% the Fsmash will kill you no matter what.