From a technical perspective, Matrix & XMPP are about as different as you can get. It's not just a marketing thing.

Matrix is a decentralised encrypted conversation store; where all conversation history gets replicated across all the participating servers/nodes. It's a bit like Git, or NNTP (Usenet) or possibly IMAP, depending on how you squint.

XMPP is a message-passing protocol; where stanzas get passed between servers/nodes. It's a bit like SMTP or SIP.

There are a bunch of other differences philosophically (Matrix is one big monolithic versioned spec; XMPP is a cloud of XEPs, etc) too.

The only similarity in the end is that you can use both to build chat systems (and you can use both to build a bunch of other things too - e.g. IOT use cases).

Another way of thinking of it is SVG versus Canvas. Or OpenGL versus a Scene Graph library (Open Inventor, or whatever that's become now). Sure you can use both to draw pretty pictures, but architecturally they couldn't be more different.

XMPP vs Matrix is a really interesting discussion. I don't think marketing is the main difference though, I think most developers would prefer to work with Matrix and the user experience is much better. Here's a decent past discussion that also links to older discussions:

https://news.ycombinator.com/item?id=12880860

I’m just not “in the know” to see if this is the case, but I just found myself taking a look at the site and thinking “what’s new about this?”