If the snowy trail has been groomed, or has a relatively smooth surface, then, yes, it is poor etiquette to run on it. Footprints harden into bumps and severely detract from the riding experience. Snowshoes are fine, in fact encouraged, because they help maintain a nice compacted tread. I'm guessing that whoever spoke to you today meant for you to keep that in mind in case we get snow, not that you shouldn't have been there today, as I can't imagine you were affecting anyone else with our current conditions.
I'm sorry that you had a bad interaction. One of our challenges is that we are all volunteers, and we don't have uniforms, or badges, or necessarily great communication skills. Sometimes when we try to do what seems right and protect the trails, it comes across as pushy or rude. Yes, we need to get some winter trail etiquette signs up. But thank you for appreciating that whoever you spoke to was trying to help.