He only latched on to Shukaku because he still had Shukaku's chakra in him. Think it was corroborated by Kishi as well in a db.
OT: He might be.. Gaara says Shukaku never initiates contact meaning Gaara has probably been the one to start up discussions and if we say its referring to early Shippuden then it would seem weird for Gaara to only say its been a while.. Not to mention how would Shukaku be able to contact Gaara in a Jin mindscape?
Then again, Urashiki says there are only two Jins so i might be completely wrong lol... Pretty sure we'll figure it out next episode anyways.