It's the US Apple website, what do you expect? Of course the measurements will be in football fields and eagles/freedom units.
Go to other local websites and the specs will be in the local units (most likely metric), but even in Europe, display sizes or car wheels are almost always denominated in inches.
If it is the US website I'm just asking where the international english website is I guess. Choosing UK or South Africa doesn't make the inch go away.
This is totally fine for companies targeting US domestic markets I guess but I suspect Apple sees itself as an international company and they should have the budget for proper internationalization.
Maybe they just don't care for people to know how thin the Air is, but the number of search results for 'thin' on the page let's me think otherwise.
Go to other local websites and the specs will be in the local units (most likely metric), but even in Europe, display sizes or car wheels are almost always denominated in inches.