To an extent, I agree. But I also disagree in some areas.
It's better to be good at something, instead of mediocre at everything
That's definitely true, but isn't it even better to be great at a lot things?
My point is, if you have the ability to learn a lot of things really well, go for it! It's good to have a lot of tools in your toolbox.
And also, by learning a wide variety of things, you can learn other things that will help you that you wouldn't have learned otherwise.
For example, I started out learning JavaScript. I learned JS really well. But I was so obsessed with the "vocabulary" of the language that I didn't understand how the grammar of programming languages applies in other languages the same way.
9+ languages later, I feel confident that I could jump into any modern coding language, and with the help of Google I could use it just as well as any other.
Also, just to be specific to this case...
I think you can justify most of this guy's learning:
I don't think he should stop learning, I think he should keep learning! Maybe pick up Java (or Kotlin) and Swift (or Objective-C) and start making mobile frontends for his web apps.
Depending on how you define "knowing" a language, you could probably say that I don't know Swift. I don't know all of the syntax and functions attached to data types. Yet I'm coding an iOS app all the same, because I've used enough languages to jump right in.
Whenever I hear about a new language or framework, I want to try it. I've used HTML/CSS/JS, React, React Native, Go, Dart, Flutter, Java, Swift, Scala, PHP, SQL, jQuery, Ruby, Rails, C++, Python, and more. And even though I'm not perfect with all of these, I've learned new things from all of them!