Not everything uses Semver (which is primarily aimed at software libraries). Blender's versioning scheme is consistent and reasonably well documented.

Yes, the rare case where you can sort version number strings alphabetically, not numerically (split on .)

And a max of 10 minor versions per major version (2.70-2.79)

They're not the same, but 2.80 is the first release in the 2.8 series, so this is the release of both the specific 2.80 version and the 2.8 series. Next will be 2.81, etc.

(Except it's not, because it hasn't been released yet. Still in beta.)

Because they are not the same version number. See https://www.blender.org/download/releases/. There does not look to be a 2.8 based on your description because it would have been called 2.08.

In Blender versioning if they are talking about single digit minor release numbers you always have to add an imaginary x at the end

2.4x branch, 2.5x branch, 2.6xbranch, ...

Every time they move the first digit after the comma something major changes.