That's almost impressively dumb

The "I" refers to the guy OP was talking to.

But he's saying it, I really hope you're joking.

Here's what I understand:
OP noted that there's a "U" in stupid. The guy OP was insulting fired back saying that there was an "I" in stupid too. So both OP and Guy agreed that Guy that was stupid.
I'm so confused what you guys are talking about.

"i" and "u" have to be said from the same reference point for the insult to work. In this case the reference point is OP no matter who else joins the insult.
If the reference point keeps changing, everyone can just deny it and say that "u" refers to OP instead.

I disagree but whatever bro