Hi @jlat-newton,
An activity would have to be associated with the contact and the deal, correct. If the task is associated to the contact only, then it would not update the deal property, unfortunately.
(I agree that there should be an easy solution to this.)
There is a related request in the HubSpot Ideas section of the community which you could comment on and upvote. The product team reviews these requests based on popularity.
Best regards!