But what is it differentiating? And is it really the best evidence of willingness to prepare? My MSc and BA on the topics, my open source contributions, two decades of industry experience... Those aren't evidence of not only willingness but execution of preparation?

That they would ask me to prepare for that is a signal as well.

In no case is it a useful signal on if I can do my job better than someone else. Some people like this type of problem and are good at it anyway which is a good signal compared to average - but there are also above average people who don't enjoy this type of problem and so don't practice it. Note that both cases the people I'm talking about did not memorize the problem and solution.

It also means "I don't have money for food, and at this point I am desperate".

That willingness to prepare doesn't reconcile with the realities of parenthood and all of the other responsibilities someone in their thirties may have. Consistently finding that time will be a huge ask, especially if you haven't worked on those problems in a while.