What exactly would require a Linux sysadmin to be able to articulate the differences between symmetric and asymmetric encryption? Some probably could, but I'd guess most couldn't give you more than a basic one liner.
As long as they have a basic understanding of how SSH, GnuPG, and SSL function and a thorough understanding of how to use them that's probably the extent of the cryptographic expertise required.
Just because it's not required to do the job doesn't mean a curious sysadmin shouldn't know the difference. The difference between symmetric and asymmetric encryption is a pretty basic part of cryptography and if you read about cryptography at all you should know it. I think it shows a lack of interest to not know more than the minimum required to do your job.
Well maybe they were more curious about filesystems and wrote their own? Or they know all about event-based programming but you didn't ask about that because you don't know anything about it?
I think it's wise to avoid "grading" on specific technical questions about things that aren't actually required for the position.
That doesn't mean you shouldn't look for bonus knowledge, but I don't think it's good to be too specific about what it is.
As long as they have a basic understanding of how SSH, GnuPG, and SSL function and a thorough understanding of how to use them that's probably the extent of the cryptographic expertise required.