I think it just means that Miska (@jussi_laako) doesn’t test against Windows Server and is unable to provide troubleshooting advice in relation to any issues that do arise. There are many people (including me) who use Server R2 with AO optimisation running HQP quite happily. I use Server R2 in minimal server mode in order to have access to the HQP desktop.
I thought the question on CA was odd because it referred to Server in core mode (not to be confused with a Roon Core). My understanding is that in core mode there is no GUI, which would be one good reason why drag n drop doesn’t work !