Notice that when he's hit normally, he's pushed back a bit. When I use the whistle-armor, he isn't pushed back at all, and the actual whistle doesn't come out 'til the auto-reticule ends.
Super armor causes the character to not flinch, so that's why if Olimar gets hit during whistle during the super armor frames, he does not get pushed back.
That said, it looks like the projectiles induce enough hitstun on Olimar and come fast enough such that the armor frames are "extended" enough to last through the whole attack.
Is anyone testing out red Uthrow to double jump purple Uair? I've been messing around with it on Training mode, and I was able to get it to register as a true combo. I think it might have potential as a somewhat reliable KO confirm. It may be more useful than red Dthrow to purple Uair because:
1) It KOes around 15-20% earlier than the Dthrow combo because Uthrow launches the opponent vertically higher than Dthrow does. As a comparison, with the training dummy set to Mega Man, the Uthrow combo can KO around 105-110% compared to Dthrow, which KOes around 125-145%. Dthrow probably has a wider KO percentage range since pulling off the Uthrow combo is somewhat strict.
2) Uthrow launches the opponent more vertically than Dthrow; I think it launches straight up with no DI. If I'm right, then it's harder for the opponent to DI away from the followup Uair unlike Dthrow, which launches at a more horizontal angle that allows for horizontal vectoring. This means that Olimar doesn't need to move around too much to get the followup, which is good considering his mobility.
That being said, there were a few problems that I found with this that make me feel like it's only somewhat reliable.
1) Just like with the Dthrow combo, it's somewhat dependent on character size, fall speed, and weight. Some characters are easy enough to get the game to register the Uthrow combo as true, while others are impossible. A character like R.O.B. is easier to followup with Uair--KO'ing at ~99-100%--because of his large frame. A fast faller like Fox can KO'ed starting around 82% despite his small frame. A tall person like Lucina can be reliably comboed around 90-95%. Captain Falcon can be KOed around 105-110%
Some characters seem impossible to register the combo as true, like Peach (too floaty), Kirby (too small and floaty), Rosalina/Mewtwo (too floaty despite their tall frames), and Mario (small frame, also Nair), but this could be remedied by making Uthrow stale. Uthrow can still be used in the same way as Dthrow at low percents since you can Fair out of it, so there are multiple chances to stale it. This brings up the next point.
2) Using Uthrow as a combo starter/KO confirm weakens its potential as a KO throw. Make Uthrow stale makes it easier to combo into Uair, but it also may weaken blue Uthrow enough that it'd KO later than it usually does.
3) Doing the combo correctly is a bit strict. You need to double jump at the right time so that Uair will barely hit the opponent. In my testing, I sometimes followup with the Uair a little too late, and it doesn't register as a true combo. This point ties in with point 1 due to how character traits influence combo reliability, but it also ties in with the next point.
4)
The purple Pikmin won't always follow up with you as you jump. I'd say this is the most important thing to consider when using this combo because it makes the whole combo a bit unreliable in the first place. Sometimes, after you use Uthrow and jump, the Pikmin will still stay on the ground for a moment before they follow up with you. Thus, when you use Uair, Olimar might be just swinging empty air instead of a purple in his hand. When it occurs is random, but it happens often enough that it hurts the viability of the combo. I don't know how to exactly fix this problem, as it's a problem with how Pikmin are programmed in the game, but it may be because I'm testing on the 3DS version.
EDIT: It seems that doing pivot grabs seems to make the chance of the Pikmin not following you less likely. I've been able to almost constantly get the Pikmin to follow me for the Uair with pivot grabs.
Despite it's potential problems, I think the Uthrow combo is useful enough to start using Uthrow as a combo throw, because it gives Olimar another semi-reliable KO confirm. I haven't really thought about the potential of this because I brushed away the idea of using Uthrow as a combo throw.
Any thoughts/tests is appreciated since this is still mostly theory.