I absolutely guarantee that most programmers I work with "believe" most of these things.
However, I think these lists are using the word "believe" to mean "incidentally assume in implementation". It doesn't require a conscious belief to write code that makes these assumptions.
Actually it's a good test for seniority in programming. In school they hardly ever teach these factoids. Yet in practical everyday programming you eventually run into some of these issues. Even if you are not a maintainer of a tz library.
However, I think these lists are using the word "believe" to mean "incidentally assume in implementation". It doesn't require a conscious belief to write code that makes these assumptions.