That's a perfectly valid stragegy. Nothing unfair in that. Your opponents made two mistakes, first they didn't scout it out that you were going to ghetto gunship them, and second they didn't have interceptors.
Don't feel like "never doing it again". Just go for it if you see a change.
It's both fair and fun IMO so don't stop trying it when you can. Hopefully it'll teach them to actually build some AA in future games, interceptors are much cheaper and faster to build than transports after all and if they'd bothered to build some air scouts as well they could have seen it coming.
I don't know what the other guy thought he was doing making more land factories and Manti after you'd already taken out his ally.