Notice the [AUR] tag, that means it's in the Arch User Repository, not in the official repository.
https://wiki.archlinux.org/title/Arch_User_Repository
https://wiki.archlinux.org/title/Official_repositories
Slightly ironic given the whole "confidence" side-note you made earlier...
https://wiki.archlinux.org/title/MATLAB https://wiki.archlinux.org/title/Scala etc.
If rustup will be removed from official repo we can argue that rust isn't supported in arch?