This is true.IMO if I’ve an idea for a Gnomish culture that fits really well with what’s established, and the DM can’t handle the thought of even discussing it, the DM is being overly precious about their vision, and need to get over themself.
It is also true that the DM might have ideas that aren't in the player-facing documents, and your idea might clash with something they want to do--whether it's with gnomes, or a thematic thing, or a geographical thing. If a player can't handle that, I'd say it's not just the DM who's being precious, here.