A validate rule is used to restrict user inputs by comparing the entered properties with a set of conditions. While an activity is to manipulate and process certain sets of action and is complex in nature.
As per my knowledge, only utility functions are allowed to be called in the "Validate" rule.
When the system runs the validate rule, it first evaluates the validation conditions defined for the first property in the list, and then it evaluates the validation settings for the second property, and so on. When a property is found to be invalid, the system ends its evaluation if no further validate actions are specified. It's basically a conditional evaluation of two properties.
I do want to understand your business requirement, why you want to call an activity in a validate rule?