If you just assume that the sample mean = the population mean, then you get the right answer, at least for this example. I don't see why the article fools around with the maximum at all - isn't the maximum a much more noisy statistic than the mean?
The range matters – had they found 10 serial numbers between 100000 and 101000, would the mean still be a meaningful estimate of the production rate? In this case, the author just tacitly assumes the minimum to be zero.
