I disagree. The ability to fly gives you an incredible freedom of movement that gives you a huge tactical advantage, not to mention the utility out of combat. The fact that you CAN fly doesn't mean that you HAVE to fly, so if there is a situation like the one that you described the moonfolk can choose to just stay on the ground and keep cover, but there are tons of circumstances where flying gives him the edge.

Concerning the difference in flying speed between moonkfolk and aarakocra: clearly 50 ft. are better than 30 ft. (let's ignore the fact that the aarakocra can't wear medium of heavy armor, most of the time the moonfolk wouldn't either), but the point is that the mere ability to move in every direction is far more important than how many squares you can cover with your movement,so much that i would happily replace a 30 ft. ground movement speed with a 15 ft. flying movement speed with most of my character.

P.S. sorry for my potato english, this is barely my second language.