Hmmm, on the other hand, suppose you first find a tank with the serial number 1234.
Then the next 50 tanks you find are all from the range [1, 100].
Is it more reasonable to assume that there are around 1258 tanks, or that there are probably closer to 100 tanks, and that first one with the very large serial number was not a sequentially numbered tank?
But, from the article's initial proposition - "You do know that the Germans have a sequential numbering system (1, 2, …, n)" and in giving historical context "On investigation, it became clear that the serial numbers were sequential, without gaps."
So, yes, without that being a prior, of course it's more likely that that outlier is a strange one off, and you'd do better to exclude it from your data set (and/or continue to investigate, because it's NOT at all clear that the serial numbers are sequential yet).
But, that context and ordering matters. Assume just the opposite series of events - you started by finding 50 tanks with serial numbers [1, 100]. And then three or four months go by you didn't get any tank serials sent to you. And then you get 1234. 1258 tanks seems really reasonable at that point (and, in fact, would fit the reality; the Germans were producing ~256 tanks per month per the article).
Then the next 50 tanks you find are all from the range [1, 100].
Is it more reasonable to assume that there are around 1258 tanks, or that there are probably closer to 100 tanks, and that first one with the very large serial number was not a sequentially numbered tank?