My "NO LAG" was an intended hyperbole. Earlier in my post, you can see that I said Diddy's upB has "low lag". Anything 10 frames (WL lag) or less is extremely short. Regular landing lag (just jump up and land without doing anything) is somewhere from 2-4 frames, I believe.
My argument about the edgeguarding thing is this:
Usually, when a characters goes out and botches an edgeguard (or in the case that I presented, two consecutive attempts), the opponent can get back before the edgeguarder and punish him for trying or at least recover successfully. Diddy is different because his upB goes so far (it still has decent distance, even with a tap) and has such little landing lag, and a lot of the time, the opponent is actually taking great care to
avoid being around Diddy because the upB comes out extremely quickly, it covers him really well, and it kills, leading to Diddy usually getting back before the one recovering, even after a total fail. The act of going offstage for a gimp is supposed to be high risk, high reward. Diddy, in my experience, turns it into low (read: no) risk, high reward.
Yes, the same could be said of Sonic, Pit, and Jiggs, but the major differences comes in how good those characters also are
on-stage. Diddy is a beast who can pretty easily switch from defensive play to offensive play on a dime and he excels at both. Sonic, Pit, and Jiggs fail in that area in some way or another. Sonic has no priority, Pit I'm starting to feel suffers from frame issues (not sure why, though), and Jiggs can't just go in or she'll get blown up. Diddy's on-stage presence is already really good and you gave him a good recovery on top of that. Don't forget that I played the same 'old Diddy' that you did, and I honestly thought it would have been fine if he kept a somewhat mediocre recovery in exchange for his excellence on-stage. The spacees are also pretty "free" to edgeguard, but only if you can get them offstage in the first place.
Admittedly, the upB is just one of a whole bunch of traits that, together, make Diddy extremely darn good. If you asked me, I would say that the combination of his sideB and his upB make him almost too good. Maybe throw his downB into that mix. They're his "shines" so to speak. To be completely honest, I don't think Diddy has any bad moves (which is good) and I'd say that his worst would be........I guess nair. That move's pretty janky.
Now, I'm aware that my opinion of 'good balance' is different from what the PMBR's seems to be. I think every character should have great strengths but clear weaknesses that can be taken advantage of (aka High/Mid Tier). The PMBR (from what I can see) wants everyone to be spacee level (Top/High Tier). If that's the case, I think you've succeeded (as you did with Lucario and possibly Ike beforehand). I'm saying Diddy's broken, but if the goal is to make everyone broken, then great. I just don't think some of the actions taken so far have actually been working towards that, even with 'sensical design' in mind.
But you know, to me it doesn't really matter. Just as I did with Lucario in 2.1, I'm picking up Diddy now. I recognized the really good stuff and the potential he has, so I'm going to use him. If he stays this way, that's great. If he changes, I won't really be surprised.