Einstein’s assistant was already cleared as high as was Einstein.
I was curious, so I looked it up. According to this article, Einstein was denied a security clearance back in 1940. Did he get one later? The article doesn’t say.
https://www.atomicheritage.org/profile/albert-einstein
That’s not how real intelligence works. You don’t show everyone with clearance, even if the assistant had clearance (which they probably wouldn’t). Everything is on a “need to know” basis. They may have needed Einstein’s opinion on something, but not his assistants’.