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.