I understand the guideline like this:
if an item has stylistic choices that a large number of users don’t like (e.g. Fragmented Robloxian v1), they would be open to changing it as long as the request was submitted within 6 months.
Here’s a good example of a stylistic issue that they didn’t change because the item was already well-established: Kidomaru the Cursed Blade accessory's particles do not properly follow accessory while in motion - #3 by Klaugrana001
For actual bugs, where the current state of the item does not match what Roblox intends, such as a texture not loading, they would fix it regardless of how old the item is.
Maybe in this case, Roblox intended for the horn particles to be symmetrical and considered the prior state to be ‘bugged’. Or maybe they saw that the report was submitted just 5 days outside the window and decided an aesthetic change would be ok. Ultimately, they have some discretion if they think they can make things better for the users.