It is clear by the mention of "several orders of magnitude" that we are talking about a database which fits in a few MB of disk space on a machine with a few GB of RAM.

If the interviewee has never worked with a database which would fit in the memory of the machine, and if the company only deals with such databases, then it is clear that it was not a good fit. Failing the interview does not mean that the interviewee is bad, just that they don't know how to count that low, and thus were not the person for the job.