I may have spoken too soon. The tools were installed (apt install xe-guest-utilities). But both XCP-center and XO from source tell me that the tools are not detected.
Do not install the guest tools from the repositories, it's completely broken, and if you do, then removing it is very difficult. I had to wipe everything and start from zero to get this right. But I can confirm that on a completely vanilla Ubuntu 22.04LT install the guest tools on the ISO that comes with 8.2 work just fine.