I'm pretty sure the A2A Mustang will do this quite well (as I've gotten the P-47 do to it), and appropriately shorten the life of your engine as a result. Not to sound harsh to this pilot as I don't fly the Mustang, but I do know that his extended attempt to run the engine the way he did is not good for it. I don't know his checklist, but whenever I've seen starts like that in person, it's been the failure to use and follow the proper starting procedures that's led to this. Following the procedures either resulted in a start or not, not this "in between" which isn't a good way to run an engine.
Dudley, do you know who that is? That's not Steve or Stevo, but it's Steve's plane...
Don't recognize him, but this can happen in the 51. You start on the primer and it's electric on the D. Depending on how hot or cold the engine was when the start was attempted, you can under prime or over prime a Merlin easily. It only takes an extra second or two on the primer switch to do it. As well the spark advance is linked to the throttle and it's easy to have it a tad too far forward at start.
Looks to me like it might have been a tad over primed and the mixture selected into "run" flooding it out. Notice the rich exhaust after the start as the mixture sorted itself out.
It has happened to a lot of of us really. The 51 is a "sensitive lady"