Just received confirmation from their support team.
“When you say HS Activity Score are you referring to the HubSpot Score? If you are trying to set the score by workflow, then that is currently not possible. The HubSpot Score is set by Lead Scoring rules that adds points automatically to the property based on criteria that you set (how-to reference).”
The problem with HS scoring itself is the criteria based system. A score shouldn’t receive points based on what current criteria it matches, it should receive accumlative points based on which criteria it has ever met, and then allow score resets.