Perhaps I haven't seen all the MS ads, but the only ones mentioning the Air that I've seen are the ones stating "Lighter than Air."
When I read that, I know what they are comparing against and I interpret that as a marketing campaign to tout the SP3's light weight--especially since there were so many harsh criticisms regarding the weight of the introduction of the original SP. I don't see that campaign being about anything more or less than just the weight of the devices, period.
If you're going to compare two different architecturally-based devices, then at least make sure that both are equivalents on their competing architecture. That is, when Apple makes a MacBook Tablet/Laptop, then by all means go ahead and compare against the equivalent Intel-based device(s) from MS (or whomever).