It started out silly and whimsical. The casts are just having fun promoting their recent success which is #Avengers #Endgame. The lyrics as they sing it are done quite nicely, to be honest. But out of nowhere, a fitting tribute to close in on the original song's title.
They sure didn't start the fire, that's for sure. But the legacy will go on. His legacy will live on and on.
Source: The Jimmy Fallon Show