Crude, but combine companies using with Github stars and I'd say the claim is accurate.
Second most used FP <> JS language would probably be Scala.js, which, as far as we know[1] doesn't yet have many companies publicly using it.
Take it all with a grain of salt, OP himself says, "ClojureScript has almost certainly become the industry's most-used functional language that compiles to JavaScript. Admittedly, this is like being the most popular person who can speak Klingon".
Translation: combined adoption of niche languages <> JS is still miniscule.
Elm in itself is useless; the claim here is that among FP server-side languages that compile to js, CS leads the pack.
Sure, with Node.js on the server then Elm, Purescript, BuckleScript, etc. can do client-server, but they can't do so in the same language, seamlessly share code, etc. That's one of the selling points of one-language-to-rule-them-all (i.e. target js, wasm, native, clr, jvm, ...).
Saying that, looking at BuckleScript this weekend -- completely shocked that there's zero overhead wrt to generated file size; it's like typed Coffeescript a la OCaml, unbelievable (this from a dev using Scala/Scala.js in day job).
Second most used FP <> JS language would probably be Scala.js, which, as far as we know[1] doesn't yet have many companies publicly using it.
Take it all with a grain of salt, OP himself says, "ClojureScript has almost certainly become the industry's most-used functional language that compiles to JavaScript. Admittedly, this is like being the most popular person who can speak Klingon".
Translation: combined adoption of niche languages <> JS is still miniscule.
[1] https://www.scala-js.org/community/