I think this is only going to increase for the foreseeable future because to get the most out of each platform you have to use platform specific languages (usually). Swift for iOS, Kotlin for Android - then whatever you use on the backend and web frontend, etc. You can use things like Flutter for multiple platforms, but they’re not quite there yet for everything (though Flutter is definitely one to watch - being backed by Google).
Something I have really enjoyed about learning another language is how things are done differently, and the ideas that I can borrow and apply from one language to another.
Programming language polyglotism is fairly normal. Many devs, juniors included, have cursory and workable knowledge in at least 2 languages.
Not sure what this article says is actually new. 
But yeah, having in mind there are a lot of small dev studios out there, polyglotism is pretty much mandatory if you are not working for a big corp.