It is possible to consider them different categories of things. XMPP solves the IM problem but conferences with lots of users are a bit awkward if you use a XMPP MUC server. Conferencing is to some extent a bag on the side of XMPP. You would probably be better off just using IRC.

Matrix is primarily about dealing with the issues of conferences with lots of users... I see it as more of an improved IRC than a replacement for XMPP.