It's very plausible that it wasn't Hartmann, I agree. Some pilots flew 109s that looked like Hartmann's just for the psychological impact.
I also suspect that it was a 30mm Mk 108 that downed the 51 in the story, and not a 20mm MG 151.
Hartmann did some effective work with the MG151. Though I would imagine that the one shot was just an exaggeration. In any case, if it was Hartmann, it would have been an MG151, as he didn't like the MK108.
In any case, this sounds like something that would have been mentioned in 'the Blonde Knight of Germany', however the only encounters with Mustangs that are described are his first (in which he shot down 5), and a few days later (where he was forced to bail out). That story doesn't sound anything like either of them, and in general it doesn't sound in any way like Hartmann would fly, being the 'picker' that he was.
Hartmann never really had 200 kills, he scored kills 193-202 on the same day. And I'm sure the pilot would have noticed 60 more kill bars under the 200, as by the time he encountered Mustangs over Ploesti this is what his tally was closer to (his first 5 Mustangs were kills 262-266). This is if Hartmann was even still marking his kills, I know he stopped by August '44 (by which time he had scored 300). In mid-1944 I don't think there were any other pilots in 9./JG52 with 200+ kills.... this sounds like it was a bit....
embellished 