Seems like a non sequitur to me, it could certainly be that the humidity/temperature/seed crystal are enough to ensure broad symmetry but not symmetry of fine details. Your argument is a bit like saying “when we look at the fingerprints of identical twins they are completely different at the level of fine structure, so shared DNA and a shared intrauterine environment simply cannot be the reason for their closely similar appearance at the course level.”

The hypothesis we are all told in school when we ask this question is that snowflakes are formed from “seed crystals” – these are tiny enough that they are a single crystal, and perfectly symmetrical. Why the crystal itself is symmetrical is left to the imagination; as is how it managed to store and convey the information for the specifics of the symmetrical branching to the growing flake.

But essentially, it it claimed that assuming that the various sides of the snowflake remain at the same humidity and temperature, they necessarily must grow at the same speed, and in the same ways, since all six sides of the seed crystal are identical. Magically identical by magic. But differently identical than other snowflakes. Magically differently identical.

Looking at these pictures, you can see the fine structure of the ice crystals around the gaps. It’s basically complete freaking chaos. There is ZERO symmetry in the fine structure. So that simply cannot be the reason for the coarse symmetry.