There was definitely at least some progress on the problem. I get the general sense that there were potential counterexamples that were close but not quite enough, and that its possible (or even likely) that Claude built on those in order to construct its solution. I also get the sense that when Zhang was working on the problem it was believed that it would be proved true, but since then there were bounds found on the problem that pointed researchers to believe it was false. I am not a research mathematician in this field though, so I could definitely be wrong.

Also, in fairness to Zhang, I believe the dissertation he ended up writing was focused on the 2D case in particular, which is still unsolved (the counter example is only for 3D and above). I cannot imagine that anyone looking at the 2D problem was not also looking at the general case as well though.