It's a perfectly valid and normal reaction. Regular viewer has no reason to suspect that this is anyone special speaking. That comment was indistinguishable from a random thought of a random person, so it was entirely reasonable to request a citation. "That's me, Rasmus" would be enough of a citation of course.
Indeed! That's what I was implying with "based on your comment history".
It required me being curious about why a random commenter would make such an authoritative (but not backed up) statement, remembering PHP's creator's name and making the connection that their username may be related to it, then reading his Wikipedia page and digging through his comment history to find a couple from 2015 (about UWaterloo) and then 2013 (about his age!) as supporting evidence!
