I worked in Facebook briefly, and the reason why Dan (who also is/was at Facebook) might be suggesting this is because this is the practice at Facebook: all "JS" files use the same build system and file extension. Which is to say, there's no JS, or TS, or even CommonJS APIs. It's all and always Facebook-flavored Flow (with the occasional caveat).
And perhaps this is why it didn't seem obvious for Dan (I'm speculating) that anyone would want to use multiple file extensions for things, but when you're outside of Facebook's ecosystem and you have to set up your own build tools and deal with all the pain it comes with, you realize stuff like this makes no sense. Because ya know, maybe targeting by file extension in your build system would make it easier to use the right transpiler (e.g. that's what Parcel does). But that's just been my perspective being in both worlds.
And perhaps this is why it didn't seem obvious for Dan (I'm speculating) that anyone would want to use multiple file extensions for things, but when you're outside of Facebook's ecosystem and you have to set up your own build tools and deal with all the pain it comes with, you realize stuff like this makes no sense. Because ya know, maybe targeting by file extension in your build system would make it easier to use the right transpiler (e.g. that's what Parcel does). But that's just been my perspective being in both worlds.