It's a joke, patterned off the famous "Estimate how many golf balls fit in a schoolbus" interview questions that are just windows into how candidates would think about such things. (They're terrible questions for engineers. Don't ask them.)

The dual purpose of the joke is that, if you played it straight, you'd probably come up with a number in or near the millions, which I am suggesting is a reasonably achievable result from comparatively trivial optimization work applied at scale. (That's sort of a thing for me.)