I've set up a test-environment and indeed it worked. Commvault was (is?) checking for specific version/agent string when connecting to XS/CHV/XCP-ng and in older versions refused to connect to XCP-ng.
I successfully installed the agent on a proxy-vm (it uses a proxy to ro-mount the VM-VHDs and backup the content), connected the pool, delivered the VM-inventory and also ran a successful backup:
171c0956-e3e2-4fb9-a93b-48a2be6751bb-grafik.png
In other words: If you're using Commvault (11.28+) you're not any longer locked on Citrix.