It's not that difficult for people smart enough to get a FAANG job. Personally, I've heard anecdotes from people who can't understand leetcode problems even when given the solution. Professionally, I've seen multiple people who were clearly trying to recite a memorized solution for an interview problem, but just weren't clever enough to do it properly or talk about how it works.
For all the complaints about whiteboarding, it does filter out a lot of people who can't seem to generalize problems or are weak at either problem solving or coding. I don't see how anyone who has given technical interviews could say whiteboarding is a waste of time, unless they are not good enough at whiteboarding to be able to productively give such an interview.
