the naive solution would say, hey, this whiteboard stuff is a synthetic benchmark. just use a real one! (ex: solve a coding problem with access to the internet)
but for large companies (and this applies to school applications as well), it is not merely a process of evaluation, but of filtering.
there are more qualified candidates than there are positions. and thus whatever process is used to reduce this number will always be somewhat arbitrary
perhaps it would seem less "unfair" if it were random, but is it really?