I don't know enough about GNOME's history, but I wonder if the people making those promises fell into the trap of believing that a more polished desktop environment would be one with cleaner code, and when they said super-polished they meant clean abstractions.
A super-polished UI (in the sense of "well-rounded, meets expectations, minimizes user surprise, behaves predictably") has messy abstractions because humans are messy. I've never met one that looked good on the outside and didn't have ugly snaggy bits on the inside to get all that good-lookingness to actually work in the corner cases. Perhaps GNOME has fallen into the trap of sacrificing features for purity of form. If they're saying things like "We'll have thumbnails when someone can figure out how to do it cleanly" they've fallen into that trap.
A super-polished UI (in the sense of "well-rounded, meets expectations, minimizes user surprise, behaves predictably") has messy abstractions because humans are messy. I've never met one that looked good on the outside and didn't have ugly snaggy bits on the inside to get all that good-lookingness to actually work in the corner cases. Perhaps GNOME has fallen into the trap of sacrificing features for purity of form. If they're saying things like "We'll have thumbnails when someone can figure out how to do it cleanly" they've fallen into that trap.