Why would an airport be built just for the gyrocopters? These should have been there before the downfall, so would be built for normal planes. If you wanted to say it was built after the fall specifically for gyrocopters, that's fine, but you wouldn't have a tower or terminal and if you had a hangar, it would be small. I mean, I did build a runway at my base with runway lights and a tower just because I felt like it, but it was hardly realistic to do so.
You aren't going to land many planes on a runway under 400m other than something like a kit plane, which would be a similar idea to the gyrocopter and might have been in someone's yard, but not set up like an airport.
As to why you can't use tiles... There is currently no way to control tile placement other than manually adding them to a map. If they added a way to control tile placement in RWG, then this would be an easy thing to do. Right now, you can't even have more than one type of gateway tile, meaning you are going to have the same kind of tile on your airport as on your towns, which will look odd.
No one is saying you can't make an airport and put it on your map manually. You have shown it can be done. It just isn't all that good an option in randomly created maps right now. I think adding control to tile placement would be a great option as it allows for far more than just airports, but we will see if they agree.
And I think that if TFP makes their own airport POI, it needs to make sense realistically and not just be an unrealistically short runway.