You can see here that I have built two airfields - the southern one on my home continent; and the northern one (above Mohenjo) on a neighbouring continent. Spoiler : I did get a boost - but not the one to Combined Arms as shown here. Spoiler : I got the boost for Rapid Deployment. Hence why I went and created another Airstrip on my home continent thinking that (even though it's not explicitly stated) maybe they have changed it so that it must be an airstrip placed there so you don't get both boosts with one improvement. Spoiler : But no. Game save attached.