It’s pretty clear by the amount of change (and Apple’s marketing efforts) that each OS 10.#.0 release is a "major" OS update, on par with the relative significance of each iOS #.0 release.
But Gruber may have over-generalized here. Indeed, Apple has added major new features in iOS .# updates, e.g. AirPlay and AirPrint in 4.2, or the Personal Hotspot feature in 4.3.
What they haven’t done is change key functionality in a point update, let alone introduce regressions. (In fact, before this incident, I’m struggling to recall Apple ever removing/crippling existing functionality.) If they were going to perform a full Maps switch-out, maybe they felt that now would be the best time, if only because there is so much positive press out there to temper the backlash.
Edit: Regarding removed features, I was mostly thinking of Apple’s record with iOS. As gurkendoktor points out, they have certainly removed ("simplified") functionality in Mac OS X, often creating great frustration.