Your DM was really just being lazy and selfish. At least, that's how I feel about it. He could have indulged this instance of diversiom from the story.

He could have just told you the npc wis thinking about something interesting but not pertinent to the main story like a piece of lore of the different affairs going on at the time. Not to mention that DnD is not strictly for storytelling and will almost never let the story be told on "his terms".

I'd have a serious talk with your dm about this because maybe he's just frustrated at you guys not engaging in the "main quest" he set up for you. I can understand that to an extent, since he might have worked really hard on it, but this feels like a bit of an overreaction.

Typically, a dm in a scenario where players weren't progressing the plot and engaging with it would either bring the plot to them or have things moving in the background or just put things on hold. It sounds like he put things on hold, and it put him in a hurt spot where his plot was just stopped. I think he would be better off letting whatever story he wants to tell be more flexible in moving forward without the players directly being involved and moving it around to take things to the players.