Alright, after doing some corpus search,
Cinefactus, I found out that the reported question "quis sim" is very rare (a few times in classics, never in Plautus or Terence), but what's more: not just Plautus and Terence ask indirectly "qui sim" with a hinted sense of "quis sim", but even classical authors, Cicero too, ask this way and it truly seems as though they mean "quis sim".
So, it may be that in the reported question "quis" simply has this phrasal variant...
It's also possible that if one read all the huge definitions of "quis" or "qui" in OLD, they would find something along these lines right away, but, alas, I've been too lazy to do that yet