When a compiler sees an import to a module with “use client”, it substitutes it with this reference. So the server code only sees the reference. Whereas the compiler also takes this as an instruction to bundle the code from that entry point. (Which ensures this reference actually corresponds to a file that will exist.)
Does this make sense? Like I said, I might be stretching the analogy, but it’s a bit like quoting except at the module system level (and in practice mostly happening at the build time).
I've edited the post to clarify this a bit.