A senior engineer comes with years of experience designing and coding systems that took years to develop and run at scale. Given time to learn they should be just as good at solving day to day bugs but if you assign them that you are wasting your time and money on experience. If you don't value experience you are wasting your time and money on experience.
It's pretty normal to have more junior engineering staff help new senior staff learn the ropes. It's also common for those less experienced staff to learn from the more experienced people even in that situation. I'm not saying senior hires should be respected from the first second because they are over 30 or have a long resume.
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.
In practice whiteboard interviews assess this pretty well. A new senior engineer must immediately be shipping optimal, tested, and well designed code. Otherwise you'll have Junior Engineers coaching Senior Engineers and the Junior Engineers will just get frustrated and leave. Worst case, you'll have an "architect" who never learns how to actually deliver independently in the environment.