I didn't watch the film but from the conversation this sounds like a HO to me. It's the "intent" that matters.
I watched the film ... what it boils down to is that Strip made a horrible merge ... hoped he could get out of it ... paid the price.
If I was in an A20 ... slow and turning towards a P-51 to engage and he dives like a banshee ... I split-S over the top of him and he zooms up in front of me ... I would have sliced him to ribbons too (if I was that good of a shot).
There were a 1000 different ways that P-51 could have merged/engaged and had superior position on the A-20 ... way too many people making crappy merges ... getting spanked on the crappy merge ... and then crying foul.
I tell ya what ... had he merged with Leviathn in the same manner ... he would have died on the first shot (I speak from experience) ... and Leviathn would have most likely gotten a <<S>> instead of a cry of HO.
Nice shot Larry !!!