Like, screw Diddy Kong. dThrow into uAir is so overdone at 80%. uTilt at 20% is where it's really at.
Let me just assess what happened in the video.
- Rosalina and Luma land in front of Bowser Jr.
- Bowser Jr. starts his fSmash but it doesn't come out yet, it charges for one or two frames.
- Luma's uTilt connects with Rosalina having 110% damage.
- Bowser Jr. is KOed off the top
Note how Jr.'s flight curve is perpendicular to the stage, meaning that DI was not involved, so I am guessing you used the C-Stick for this smash attack.
Also note the two highlighted parts. Luma's uTilt is notorious for having an extremely high base knock back but low knock back growth. If you hit a fighter who is charging their smash attack, the knock back from the attack will be multiplied by some value, which goes for all characters. The already huge base knock back on Luma's uTilt gets multiplied and KOs extremely early. This is also seen in the old Chiko Special videos where a Donkey Kong charged an fSmash on top of Luma and he got KOed by uTilt as well. I did this on a Ganondorf, too, same situation.
In fact, I just tested this consistently in training mode. Turn the damage up to like 56% and have Bowser Jr. charge his fSmash and let Luma hit him during the first few frames. He'll get KOed off the top, but if he doesn't charge an attack, the knock back sends him just barely to the top of the screen.
In training mode, there is neither rage nor the bonus from using a fresh move, so essentially, Rosalina having 110% damage and likely using Luma's uTilt for the first time in a while caused you to get KOed at a ridiculously low percentage because it just so happened to connect during your charging an fSmash for the smallest moment.
So yeah, basically if Rosalina gets a hard read on your smash attack nearby, Luma can KO you for it if you don't release it fast enough.
Slight update: For comparison, if this uTilt hits without Jr. charging an attack, it KOs at 132%. Quite the difference.