It's most likely edge-cancelling, as shown in Jeepy Sol's video.
It's been known for a while that the spacies could edge-cancel their Side+B's and that Lucario could edge-cancel his Up+B. I was not aware the spacies could edge-cancel their Up+B personally, but it may also have been common knowledge. I also did not realize Marth could.
In any event, this is almost certainly what is occurring. You're edge-canceling your Up+B, which cancels all lag, and then doing another Up+B in that space. Edge-canceling is a handy quirk, and the ability to perform it on command would be a great skill. I haven't made any attempt to try it too much myself, but I imagine it's not particularly easy to do it consistently. There are a lot of options out of an edge cancel. Another Up+B is one, but the fact is, an edge cancel is usually going to take your opponent by surprise, so there's a wealth of possibilities you can perform on a stunned opponent from here.
If you can pull it off consistently, experiment with different aerials and others specials and really work your opponents failure-to-predict to the fullest potential.