Horizon Advanced and Enterprise Edition includes rights to use vsan for free. However in the documentation all-flash is explicitly mentioned, does it mean that this license right is only for All-Flash hardware configuration usage and cant be use for hybrid ones.
Take a look here
vSAN can be used in all-flash architecture in which server-attached flash devices provide both caching and data persistence for extremely high and consistent levels of performance or in a hybrid configuration in which server-side flash devices are pooled to provide a read/write cache and server-attached HDDs provide data persistence.
VSAN for Desktop I'm pretty sure is the same thing as vSAN for VDI, I can't see why there would be different ones, plus I've never head of vsan for vdi, other than this page I think its just a different name for the same thing.