I'd strongly disagree, In lucario's case even with aura he pretty much can't touch rosalina until luma is off her. The flow of my tourney matches has generally gone like this: Get bodied til 100% dealing almost no damage -> luma dies to something -> get rosa to 50% off of 3 hits -> rosa gets baired or smashed and dies. when rosa has luma on her against lucario she can safely apply ridiculous pressure that he can't beat out in terms of speed or range, It's pretty crazy.
I'm gonna assume from your description that you are either having trouble getting into a position to gimp Luma or you just haven't figured out how to do it yet. I don't know how to help you if it's the former, but assuming the latter I can give a few tips.
-If Rosalina is in a tumble, Luma can't move. He wont attack and he wont follow Rosalina until she recovers from her tumble. This is crucial to understanding the process of most of these gimps.
-Yes Luma can attack while Rosalina is flinching but not while she's tumbling. In fact tumbling is probably Rosalina's weakest position to be in.
-If Luma is tumbling off the side of the stage, he'll die and Rosalina can't do anything about it. At least, if she can, I haven't found a method to get Luma out of his tumble (other than Luma simply landing on the stage, of course)
-Quickest way to get rid of Luma is generally a smash attack while Rosalina is too close to an edge, and it works at 0%. Lucario's F-smash can do it uncharged, if Luma is close enough to the edge. Lucario's D-smash and U-smash don't seem to help much at low percents, but my tests at 50% show that D-smash knocks back Rosalina but not Luma, allowing you to knock back Luma with an f-smash, and U-smash can knock Luma up allowing you to follow up with a fair before Luma recovers. Be careful using the U-smash method though, as it seems to leave you very open to retaliation from Rosalina.
-How well has Lucario's Side B worked for you, in your experience? Testing it at 0% and 50%, it seems to only knockback Rosalina, separating her from Luma. This sends her into a tumble and you can follow up with a dash attack that will also gimp Luma if its near an edge. With Lucario at higher percents, even an ftilt can finish off Luma.
-Have you tried separating Luma from Rosalina with a down smash, or even just by smashing one and not the other? Again, Luma wont follow Rosalina if she's tumbling in the air, so you can launch her and, instead of trying to gimp Luma (perhaps because Luma's not close enough to the edge or something), just go after Rosalina and follow up with aerials.
-Some RosaLuma players will know about these gimps and might try to stay away from the edge. You might have to bait them or knock them there somehow, or punish them when they try to recover from the sides.
-Not all attacks that knock back Rosalina will knock back Luma. I suggest going into training mode and seeing which moves separate them and which ones don't.
-Gimping is better than Separating, which is better than Killing. All damage against Luma is wasted and the 8 second window isn't enough to justify it. Separating them will give you a window to attack Rosalina if you can get to her while she's still tumbling. Gimping is the best as it's 8 seconds of freedom for the low low cost of 1 or 2 attacks.
-In any case, while Rosalina's tumbling does give you a window to gimp Luma, it's a short one. If you can't gimp Luma right away with a single attack, I suggest going after Rosalina instead. Damage against Rosalina will always help you, but damage against Luma almost never will. (EDIT: It's possible in some situations to get 2 or more hits on Luma before Rosalina gets out of tumble, but I haven't been able to test any anti-Luma combos against competent opponents.)
YMMV on this and I suggest experimenting to see which ones work best.