First, the average quality of candidates we were getting was pretty good. She stood out, and definitely gave a memorable performance on the technical level, but it wasn't some colossal blow to our org that we didn't make the hire.

Second, she wasn't interviewing for a "money goes in, code comes out" code monkey-type role. Whoever took that role was expected to communicate with a bunch of people.

Third, the ask was "explain this to a layperson", her performance was "a senior technical person can barely keep up". It wasn't a matter of not performing satisfactorily, it was a matter of completely failing. I really liked her as a candidate, I wanted to make the hire happen, and I'm cautious about interview nerves messing with people, so I really tried to steer the conversation in a direction she could succeed, but she just wouldn't follow.

Being able to explain a thing to someone non-technical is an important social requirement. If you have to explain a problem or project to a C-level and you go off the rails with technical stuff, or get deep in the weeds of some part of it, without being asked, youre going to get deer stares and no one in the room is going to understand you. Similarly, if you as an engineer, go too technical when explaining things to an admin or jr, then you are also going to get deer stares and no one is going to understand you, or they will get frustrated.

You can be a """"rockstar"""" engineer and still not be a good fit because you cant sanely explain something to someone not at your technical level.