A dubious distinction is one that is doubtful or possibly untrue. Given the lack of hard data, the aporcryphal nature of the stories, and the fog of war etc, dubious seems like a perfectly cromulent word to use here.
IMO both usages are correct, although yours is probably more common. 'Dubious' here means something like questionable/uncertain & it can apply either way: The distinction can be 'questionable' in terms of whether it's something to be proud of, or in terms of whether it's even true in the first place.
I had a similar discussion here with someone who thought that "dynamic programming" was the act of programming in some sort of dynamic manner. Not sure I managed to disabuse them of that notion.
Not sure what’s dubious about it…