Quoting the relevant part from the transcript that Guus shared:

Nicholas: Got it. The first thing I noticed about XMTP is the name, which reminds me of SMTP. Is there a reason for the relationship? Is it a reference, first of all? And if so, why?

Matt Galligan: Yeah, it actually it's pretty spot on, but it goes one nerdy level deeper, which is there was also another protocol XMPP, sometimes referred to as the Jabber protocol. And when I was trying to think through what to call this thing, the one thing that stuck with me was that this was going to be a protocol where any kind of communication could happen on whether it be email or push notifications or DMs or whatnot. And so I did the very nerdy thing of just what are other protocols named? You know, what can make something feel immediately familiar, though you've never seen it before? And so looking at SMTP and XMPP, I was like, well, here you go. Here's two communication protocols. And I smashed them together. Doesn't go much deeper than that. But, you know, the value being like we've heard from a lot of folks that, you know, hey, I've never heard of XMPP before, but I'm assuming that it has to do with communication. So it serves a purpose for sure.

Having read more of the interview transcript, it's clear that the founder (Matt Gallighan) has used XMPP before and knows what he is talking about. There was also some thought (though minor) given to the naming, and it seems to have been chosen as a nod of acknowledgement to XMPP (and SMTP) rather than a rip-off.

I'm still not entirely happy with the similarity personally, but I can live with it, and I'm curious to hear what others have to say.

~Badri