Real interoperability is federation. If we had federation, the problem of using your favorite client would be moot. My argument is that there never was a "golden age" of chat, unless you refer to IRC back in the nineties and even that would be debatable.
And out of the big boys I can only name Google's Talk as supporting federation, only to be killed and replaced by Hangouts, that doesn't support federation because, wouldn't you know, they needed to innovate and couldn't do that while supporting XMPP, allegedly. And even if Hangouts pushed WebRTC forward, I'm glad it's not used and I'm glad that it suffers a slow death, because this proves that the innovation argument is bullshit. And I'm fairly certain that Google Talk was more popular than Hangouts, even if Hangouts got pushed down the throats of every Android user.
Personally, if I were a place like Google where the content is more important than the product. I would be having very pointed discussions with the people in charge of things like Hangouts and Allo. There seems to be a lack of vision compared to what is easily doable in this space. Knowing enough of the XMPP spec to see what was never done, I have to agree that the author is right. Google never understood what they had with GChat.
To put it another way. Imagine if everybody ran around trying to make their own SMTP or HTTP protocols? Thats what everybody is trying to do with chat right now.