To me it seems to matter quite a bit whether this was
(1) some sort of anti-semitic attack on Lee and Kirby (which seems implausible, but would be utterly despicable and inexcusable)
(2) a joke on them being Jewish (which would be a horrible misjudgment and terribly insensitive to anyone affected by the Holocaust, possibly a firing offense), or
(3) an in-joke about something else, either
(a) not considering how it would be interpreted (which seems the most likely explanation, again showing insensitivity but not as bad), or
(b) being actually unaware of the historical meaning of the term, and maybe even Lee and Kirby's background (which would be a demonstration of such shocking ignorance--the first part, not the second--that it seems hard to fathom).
I'm not saying that people are wrong to be upset. Just that, particularly when it comes to words, intent does matter.