My understanding - those Sims are mutual Best Friends, so they don't count. The 6 Best Friends have to be unique, and if they're mutual, they're not unique.
Example:
Immortal Candidate 1 has these best friends: A, B, C, D, E, F, G, H, I, J. Even if he counts A-F as BF's, G-J cannot be count as BF's with any other Immortal Candidate.