I assumed (perhaps incorrectly) that they meant non-macOS Apple hardware servers.

Also this was never intended for people using an old laptop 'as a server' anyway, of course that works, and people doing that don't need the enterprise features from a server-specific version of the OS.

Well, some people claim that non-Server Windows can't ever be used in a server role, even with third-party software, and that you need a CAL for every person connecting to third-party software on Windows Server, so it's at least possible.

And, indeed, Apple does have a similar clause in their EULA: https://www.apple.com/legal/sla/docs/macOSMonterey.pdf

> Except as otherwise permitted by the terms of this License or otherwise licensed by Apple: (i) only one user may use the Apple Software at a time, and (ii) you may not make the Apple Software available over a network where it could be run or used by multiple computers at the same time

It's of course debatable whether, say, using the built-in file sharing counts as "making the Apple Software available", doubly so for third-party servers, but that's exactly the argument some of those Microsoft licencing people are making.