I suspect that "impartial friend" is an oxymoron, except in the limited case of the friend being a friend to Ivan Kohler and you.
I've read the dialogue, and I think Ivan is right. I think he's right,
with this as the this main groundmostly for this reason: any change in behavior visible to the user is a bad thing, unless there are cases where the module is behaving in contradiction to its documentation1. ISO 7812 may allow credit card numbers shorter or longer than 13 digits, but I suspect that shorter numbers are very unlikely, as 13 digits only allows five digits for the customer number.
No, I don't think either Craig or Ivan are being "jerks." Disagreements don't imply either party is a jerk; it's the behavior during and after a disagreement that puts one (or both) participants into the state of jerk-dom. Both have legitimate reasons for their positions. Both are remaining civil, so neither is a jerk.
I've got to stop writing these when I have to leave the keyboard.
1 If the documentation says something other than 13 digit card numbers will work, and they don't, it's a bug, and Ivan should fix it. Apparently, it doesn't, so Craig was asking for an extension, which Ivan seems to consider as too likely to break existing applications.