They might as well list 'Can't talk via Skype' as a problem with XMPP, since it's just as true and Matrix is just as incapable of solving it.
As for 'requires plugins/extensions', if you think that's a problem then there's an easy fix: define a new protocol as "XMPP + the following extensions...". That requires some effort, e.g. to get servers and clients to support this new protocol, but unlike a "clean break" it wouldn't require much technical or social work.
I especially enjoyed the "no open source implementation exists" reasoning; no open source implementation of Matrix used to exist, but that didn't stop the developers ;)