Learning the ropes of a new company is fair and being slower at first is to be expected, but explaining that code reviews need to be tested to a senior hire isn't as fair. Similarly a new senior hire should be able to hop into a code review and catch problems/suggest improvements early in the process.

These two behaviors aren't as dependent on knowledge of the system as they are on core coding competencies. The whiteboard interview reasonably works to assess these, but it requires a lot of prep and is prone to false negatives.