I'm no expert in Wireguard other than setting it up for personal use at home, so don't quite know more advanced configurations, but this seems like a separate issue relatively easily handled. That said, for my own use case, I wouldn't want to restrict IPs beyond maybe nation restrictions as I expect to connect from untrusted locations.
Say you have a WG server on a VPS that your laptop and your Plex server connect to. Your laptop should be allowed to connect anywhere, but you should be suspicious if your Plex server connects from anywhere other than your home address.
You're probably familiar with this feature if you've ever used MySQL, users have a username and host spec.
It sounds like they want the system to somehow know which IP is supposed to be associated with a particular key.
You already can restrict public IPs using iptables…
I wouldn’t lose sleep over not having that. It’d be a nice bonus though.