To this day, XMPP is a bandwidth-hungry, unreliable mess of a protocol. Despite the enthusiasts having an XEP they can flash for every objection, the sad fact is servers and clients support varied subsets of XMPP, and it's a rare day when everyone involved supports all the features everyone wants to use.
The author seems to think that XMPP is qualified to compete with WhatsApp. While WhatsApp recently abandoned several platforms, for many many years they had even S40 (dumbphone) support. I tried to find XMPP/Jabber clients for S40 phones for YEARS, and every single one I found would immediately crash with OOM errors.
It's easy to blow off complaints like mine, because everyone on hackernews has a Samsung Universe S2000 or an iPhone 128 Plus, but these are real concerns that real people have in the real world. Wholly aside from network effects, even nontechnical users said "why do I need a smartphone to send text messages?" and "because XML" is not going to convince them.
XMPP is just a bad protocol, a holdover from an era where gleefully wrapping everything in XML was the new hotness. I hope we're moving past Global XML Obsession, and I hope I don't have to hear XMPP evangelism once we get past.