I agree! This is a bug as the PrimaryAttributeValueId + PrimaryAttributeValue are actually Lead Id + Lead Name.
The related Activity Type 29 (Delete Lead from SFDC) has the same issue. And in both cases, I don't know what happens to the boolean that's supposed to mean, "Also deleted from the other side."
There's also the minor unexpected behavior that a failed Type 37 appears in the lead's ActLog with an exception, yet cannot be queried. This would be expected if the Activities endpoint solely reflected successful actions, but if you compare to Type 110 it's not consistent. Again, this is minor.