Wario can punish it pretty hard actually lol. Free Uair or Fart (Fsmash too if you are closer to the ground when he does Dair and you SDI to the side instead of under it.) I think PX being modest about that.
Also, I have explained this many times:
"The reason people talk about Diddy fsmash but don't talk about dair is that Diddy fsmash is frame 12, and the second if it connects is like 26, there's a huge difference between 4 and 13 and 12 and 26, especially if the DI direction of fsmash is also beneficial for dsmash/fthrow, his other kill moves. In other words, no, they aren't the same thing. Lucario dair is more in the league of snake ftilt with a more specific hitbox, the other issue is due to Lucario's lingering boxes meaning that the window becomes slightly more tight than the average multihit move."
How fast a hitbox ends and how fast the second one starts does NOT matter in the big picture for SDI. SDI is inputted BEFORE that frame transition takes place.
Take G&W's Bair for instance. Each hit of Bair starts and ends fairly quickly. Definitely less than 8 frames IIRC. The reason it's easy to SDI his bair is because Bair leaves the opponent in a TON of hitlag (or whatever phase they are in when they SDI). They stay in that phase for quite awhile.
I think you guys are mistaking SDI as occuring while a move is transitioning. Most of the time (I'm sure there are exceptions), SDI takes place, then Hitstun takes place/the attacking character now finally starts to go into his next hitbox. So take Diddy's Fsmash for example. When the first hit is 12th frame and the second hit is the 26th frame, this doesn't mean that when you get hit by the first part, that while you SDI you pass by frame 13, 14, 15, 16, 17, 18, etc. When the first part hits you, it "lags" and SDI takes place. After SDI takes place, you experience Hitstun and only HERE does Diddy start experiencing "frame 13, 14, 15, 16, etc".
So take Lucario's Dair for example. Let's say you hit with it, frame 4. Once you hit with it, Lucario stops. While he is stopped, the other guy can SDI for quite awhile. After a certain point, he cannot SDI and this is where he experiences hitstun. Right at this point, Lucario starts to move again. So now the character is in hitstun and Lucario swings his Leg for the second hit of Dair, frame 13 (I'm taking your word for it, the numbers sound right but I've not looked extensively at his frame data.) Now somewhere around here, the hitstun from the first hit wears off. Maybe right at the second hit, maybe a few frames after. The point though is that the other person recovers from the first hit faster than Lucario recovers from the entire attack. Sure the second hitbox may come out on frame 13, but it takes Lucario awhile after that point to fully recover and be able to move. It is in this window that characters can punish Lucario, which is safe to say it is at least 10 frames for the other character if not slightly more.