I'm one of those people, but there's more nuance than that to it. I've learned about 20 different languages to some degree, from Python to brainfuck to Elixir currently. When I learn a new language, I go to the source material, so for elixir I used the tutorials at the elixir home page and their API documentation. I read the tutorials, writing each line of code into my editor, running each, modifying it to see how it works, etc. Then, everytime the tutorial links to or mentions an API, I'll go read that documentation page in full and experimenting. Then I'll make some projects with that language, look at how the more experienced Elixir (or whatever) developers code by exploring their repos, etc.
After doing that 10+ times, it becomes easier to see the patterns in languages, and easier to understand how their underlying structure will affect how you have to code. There's only so many ways to write a compiler or interpreter, and if you know how they work and you know that your language is, for example, optimized for tail recursion, then you get your experience from other similar languages as a bonus.
I see your point if you're saying, on a surface level, that, "you know one, you know them all," is wrong, but if you dig deeper I think you'll see that, for the experienced (as in: someone who's been around a while) developer, you kinda can say that, with some reservations.
I think anyone can do the same thing, it just takes a decade or more to get there. I'm kinda dumb, too, so if I can do it, anyone can.