Special:GlobalRenameRequest does not check new usernames for potential AntiSpoof conflicts.
For the time being I believe this is intentional. Since, for at least SUL finalization, we have a high volume of requests for similar names with a great unlikelihood that the account will be working on the wiki with the owner of the similar name, similar names are being allowed as long as they are not on the home wiki. I'm not sure if this is likely to change once things have died down or not.
@Keegan Now that we are out of the backlog, I think that there is value in changing this to implement anti-spoof. I can see why we did it that way prior to globalisation.
We now have situations where people are looking to change to simple target names not related to their originating name and trying to do it based on different case, etc. If we do this based on the request form that eliminates issues, and we can put help text that pushes them to [[m:SRUC]] for complex rename requests.