Apparently - there's certainly been a few instances where a phone was already a release or two back at launch and never got upgraded.

The copy is borderline deceptive tho; the footnote clarifies "guaranteed upgrade" with "The device will receive at least one software update to bring it up to date with the current KitKat 4.4.3 operating system". So the "upgrade" isn't to the next, not-yet-released Android version but only to the next point release...

is clearly something to say when they realizes that in the world of android phones it is not always guaranteed by the manufacturers