During that post-game interview he also explicitly said that he would like to be back in Boston, though the article didn’t quote that part.
So, I mean, this is natural. His first choice is Boston, but he also is a competitor and wants to win a ring while he’s still a major contributor. Don’t blame him at all for wanting to see where the team is going before verbally committing here, especially if Kyrie might leave.
That said, I don’t think Kyrie leaving is the end-all for Horford, as I think you could convince him to stay without Kyrie if there’s no other obvious landing spot for him that puts him in a better position to win.