Hi, author here. With respect, I am a mid-career senior engineer too. I trained for a few weeks before interviewing at Facebook because, well, changing jobs is important. There is zero harm spending some time warming up mentally.

I recommend that people prepare because I've seen many many obviously smart people not make it through. It's like showing up for a singing audition without warming up your voice. You might have a beautiful voice, but you have to show it in the best condition possible.

They can't test you on the intricacies of your current project, because they don't know them.

An interview needs a greatest common denominator. That is what an undergraduate CS test is.