Is it clear what the actual restrictions are here? I have a couple of diskstations and like them and was about to buy another. How does this actually affect me as a practical matter?

It depends on the model you buy. On 2025 models forward, you cannot add unsupported disks to a storage pool. If you bring an existing pool forward from another machine, it will let you continue to use the old disks, but any replacement ones would need to be Synology-branded.