They sound really worn out. Probably just in general, likely from DMing. I'm thinking having someone else DM might be a good break for them, rather than having to think more.
You did ask them to do a lot of thinking.
If they're finding the whole matter overwhelming, more thinking might provoke a short reaction like that.
You're probably right, and if it had been just this one rude message then I wouldn't be responding like I am. But it's been a pattern of rudeness. Had my DM said something like, "Hey, I'm feeling really stressed right now. Can you hold off on the questions?" then everything would be cool. But responses like the one I was sent aren't cool at all, especially not from someone you've known for so long.
Also, for a little bit of context, I can really understand being burnt out. I'm an associate at a small law firm. I've got a lot of work on my plate. I also have a 7 month old niece in the hospital with cancer. My brother-in-law and sister-in-law, her parents, are going through hell. My wife is very close to her family, and I'm doing what I can to support her and all of them.
My DM knows all of these things and yet the rudeness still flowed. Gaming was supposed to be an outlet for some of my stress. My DM made it a stressor instead.