I've interviewed people from $BigTechCompany (with 5+ years experience) that couldn't do some basic whiteboard coding question. I wasn't asking any dynamic programming-type obscure thing that no one actually uses, but rather a realistic question that they might actually have to code (indeed it was based on a problem I had to solve myself at my job).

So it's not that I don't trust that they didn't work there, but that doesn't mean that they can do a good job here. Every company has low performers, perhaps their previous employer was just bad at detecting them.