To be honest, that is also what I thought -- logs were primary AC, if I felt and believe closed logs (not open ones) also counted for all those involved in the closed log. By its nature, closed logs are set up like joint posts, and you would hope people continue to respond in those posts as they took the time to set them up.

I'd hope they are equivalent furthermore as joint posts don't automatically equate to joint responses (to comments) from characters involved in the joint post. One may wish they did, yet I've seen otherwise (and at times been guilty of otherwise) in the journal community.