Yeah my first thought was, well you could lay out the timestamp in a way that supports lexicographical sorting, and then realized they had already done so. No reason this shouldn’t be a string!
It can be both. DNS serial numbers are YYYYMMDDnn, and it works just fine for humans, it is easy to convert, more compact, and you can compare in a single operation. Internally, there is no reason for it to be a string.