If this is Apple's reasoning, then why does SteamLink on iOS exist? I can stream games to my iPhone directly from my library. Is it because I am streaming from my computer?
According to their guidelines, yes. Not that there's any way to really verify this on Apple's end but apps like this are allowed so long as they're used to connect to your own hardware.
4.2.7 Remote Desktop Clients: If your remote desktop app acts as a mirror of specific software or services rather than a generic mirror of the host device, it must comply with the following:
(a) The app must only connect to a user-owned host device that is a personal computer or dedicated game console owned by the user, and both the host device and client must be connected on a local and LAN-based network.
[...]
(e) Thin clients for cloud-based apps are not appropriate for the App Store.