As a hiring manager, I can understand why you didn’t get the job. I agree that it’s not a “good” question, sure, but when you’re hiring for a job where the demand is high because a lot is on the line, the last thing you’re going to do is hire someone who says their skills are “6.5/10” after almost a decade of experience. They wanted to hear how confident you were in your ability to solve problems with .NET. They didn’t want to hear “aCtUaLlY, nO oNe Is PeRfEcT.” They likely hired the person who said “gee, I feel like my skills are 10/10 after all these years of experience of problem solving. So far there hasn’t been a problem I couldn’t solve with .NET!” That gives the hiring manager way more confidence than something along the lines of “6.5/10 after almost a decade, but hire me because no one is perfect.” (I am over simplifying what you said, because this is potentially how they remembered you.)
Unfortunately, interviews for developer jobs can be a bit of a crap shoot.
This site is great for learning the foot notes to programming languages, including stuff like json, yaml, and hcl.