You are right that most of the examples on that dialog page ignore the punctuation rule. It could be outdated examples. That page however doesn't address punctuation so I would follow the rules described on the page about writing. There is stated:
Avoid using periods on solitary sentences within these UI elements:
- Dialog body text
A funny fact is that "Dialog" in that text is a link that refers to the top of the dialog page where the first example is the only right example.
Later the page about writing refers to a different section of the dialog page. (the one referred to by the OP). From there you only see the "wrong" (lets assume outdated) examples.
The following components have specific writing guidance: