I can't provide a solid answer with citations on this - but it does look quite possible it was actually DeNiro doing the opening shot of this stunt.
On the insurance aspect, you might have to ask Tom Cruise how he gets away with being strapped to the outside of a plane, or in the upcoming Top Gun, actually flying one!
The shot only lasts two seconds so is ...
The easiest example of this that can be done by anyone with the most basic photoshop skills is using multiple images of the same scene, and then cut biths out of those images that you want. Note: professional editors of course use many more advanced tools, but the principle remains the same.
A quick and dirty Paint example might help convey the basic idea. I ...
They use computers, and have various techniques depending on the situation:
See also: https://www.fxguide.com/fxpodcasts/the_art_of_wire_removal/
Wire Removal By Painting Frame By Frame
Rig Removal By Patching Over The Top
Stabilise and Paint Back
Roto Clone Tools/Source Nodes
2D Background Replacement
I also have no authoritative sources, but I've been single stepping through the shot on YouTube, and here's what I see.
The camera is locked down.
Although they look and act like they're directly opposite each other, the woman shooting the lighter fluid (Juliette Lewis?) is more in the foreground and De Niro is more toward the back. At first, I thought ...
Looks like real airplanes doing real maneuvers, but it looks like there is some camera trickery, editing, and/or post-production effects to amp it up a bit. Camera work can make two planes appear to be closer than they really were; post-production compositing could also make two or more planes appear to come in very close when they were never actually close ...