The problem for most people when it comes to the "Mirrored Shards" idea is that the Shards were cut off from the Source so long ago. We're talking before the 1st Umbral Calamity which was thousands of years before Allag which fell five thousand years ago. The odds of any of the Shards developing to be visually similar to the Source over that period of time is very, very low. If they were to look similar to the Source, it's more likely that it was random chance then on purpose. We also haven't seen anything of what Ilsabard or the New World looks like, so we are missing a lot of examples of what a "Hydalyn zone" looks like.

There's also the fact that buildings constructed out of similar materials look similar just because of how those materials behave. Like, I can't tell if those pictures are of Gelmora or Amdapoor just based on looking at it without the rest of the zone's context. However, both Gelmor and Amdapoor are made out of stone with pointed arches for doors since stone buildings tend to do well with the kind of design. Anyx Trine would be another example...