The article implies that the "ground-truth" (experimental determined) structure has accuracy interval as well. Above 90% is the same accuracy as what you get from experimental determined results, hence the "solved" claim.
Solving the inverse problem would be even more valuable -- given a specific shape (and other biochemical desiderata), what sequence of amino acids would create that protein?
As hard as the protein folding problem is, the inverse problem is harder still. THAT is the one true grail.
We "solved" this at Google years ago using Exacycle. We ran Rosetta (the premier protein design tool) at scale. The visiting scientist (who later joined GOogle and created DeepDream) said it worked really well "I could just watch a folder and good designs would show up as PDB files in a directory".
The protein folding problem is predicated on the idea that there is a ground truth (a single static set of atomic coordinates with positional variances). If your point is that even experimental methods can't truly reach 100% (due either to underlying motion in the protein, or can't determine the structure), that's more or less what Moult is saying (they more or less arbitrarily define ~1A resoution and GDT of 90 as the "threshold at which the problem is solved").
Does 84% == solved?
Also, any low hanging frut implications for longevity tech?