The combo counter goes off of the normal hitstun which increases with the knockback of the move and is the point you can jump/use B moves/etc. It doesn't take into consideration that on any knockdown knockbacks where you go into a tumble you are always able to airdodge after frame 13 and use an A attack after frame 25 to cancel the hitstun, regardless of how powerful the knockback was.
D-throw causes tumble on any character even from 0% because it has 75 BKB (base knockback) and is above the tumble threshold. If his throw were a little weaker and had less BKB he would actually have a legit chaingrab/**** combos because they would not be allowed to DI it or airdodge/attack really early out of it. This is why the small amount of actual combos that do exist in the game are generally limited to things with really weak knockback that don't cause them to tumble and doesn't allow them to cancel hitstun early.
Whether the knockback caused tumble and sent a moderate distance to cause 30 frames of normal hitstun, or sent you rocketing off the screen with a Falcon Pawnch at 9002% and would have stunned you for like 5 minutes normally (if a stage were big enough for you to not die well before then), you'd still be able to airdodge or attack during the knockback at those same times.
That's why something like stomp->knee will register as a combo for Falcon yet they have time to start and nearly complete an entire airdodge before the knee hits. In the case of d-throw->up-B registering as a combo, if they for some reason were to not cancel the hitstun by using an attack or airdodge, they would have been hit by the up-B during the normal/non-canceled hitstun.
It also registers weird things like trips to falcon punches as combos. Probably has to do with not going into certain animations like standing or freefalling after being hit that would cause the counter to reset.
