I do confirm, Proximity routing is not supported (for the moment) in VMC with our embedded HCX.
Proximity routing newly renamed "Mobility optimized networking" is embedded into HCX including HCX provided with VMC.
And yes, VMC leverages NSX-T.
I run into the lack of proximity routing during my PoC in early 2022. I knew it was released starting 2021 under the name "mobility optimized networking". So I ran again into this topic during my "VMware Cloud on AWS - Install, Configure and Manage" course and I was ablte to test the functionality. I can confirm its working as it should and it is easy to configure.