It means it’s a robust well-tested protocol (referring to HTTP)
XMPP by now is no less well-tested.
Average company firewall: Allow 80 Allow 443 Allow 53 to <internal DNS server> Deny to any
Average company firewall shouldn't allow 80 and 443 to outside anyway.
Anyway, that could have been a fallback, it's the only way instead.
Doing an IM over TCP I can understand. VoIP signalling over TCP is not serious.
What’s the better solution?
Look at Retroshare. In this particular regard (not its whole model of security, which is seemingly not good, but I'm not a specialist) it does things right, I think.
Yeah it has a lot of problems, but all the things you listed are the least of it.
And which are not in your opinion?
Still better than anything else.
Still not better than XMPP, so factually wrong. =)