I'm sure individual interviewers have their own styles, but yeah I'm with you here. Few things are more frustrating for me during an interview than wasting 30 minutes going in circles on something because the candidate isn't being honest with me.

Our role (low level software) is going to be full of things they haven't seen before. I would rather have a candidate who can quickly identify that they don't understand something, and likewise quickly try to fill that gap so they can move on to the next thing, than have someone try to bluff their way through.

I understand that there's a level of "fake it til you make it" during interviews, but the goal of the interviewer is to get as much signal on you as a candidate as possible. Admitting you don't know something may not feel good, but then it gives the interviewer the opportunity to test you on different things that could really highlight your skills. For example, we ask questions on multithreading during our panel. If you don't know how a semaphore works, and you tell me that upfront, that gives me the opportunity to explain the concept to you and see what your process is like working through new information.