Actually, the HPET doesn't have to be present, but the virtual hardware version does have to be 8. New virtual CPU features are typically tied to a new virtual hardware version.
In the case of virtualized VT-x/EPT or AMD-V/RVI, we didn't quite make the cut-off for virtual hardware version 8, which first appeared in ESXi 5.0, but we decided to release the feature in Workstation 8 anyway. That's what the warning is about. Future products will probably tie virtualized VT-x/EPT or AMD-V/RVI to hardware version 9. (It is possible that hardware version 8 VMs with the feature will be grandfathered in, but you won't be able to add the feature to a hardware version 8 VM that doesn't already have it.)