Self-sacrifice and very high risk of one's own life (a not insignificant number are given posthumously)

So a lot of medal of honor recipients are infantry pulling wounded squad mates out of danger in extreme circumstances.

The entire air force has only 17, the army has over two thousand.

He did risk his life when he interjected himself between the fleet and the approaching MIGs and then spent the next 30 minutes in a dogfight to down 4 of them. It's probably a lot harder in the Air Force to get one but I would consider that one.

He was brave, and did his duty. But, is this different than infantry advancing while under fire?

The highest is the Medal of Honor for "Conspicuous gallantry and intrepidity at the risk of life above and beyond the call of duty". See: https://en.wikipedia.org/wiki/Medal_of_Honor

Yeah to get a better idea of what that means there might be an example on Army writer. You have to write these a certain way and the granting authority will have their own quirks.

I imagine the comparison to gritty heroics on the ground, face-to-face, means it's harder for pilots to earn that.

GIYF, there are specific requirements for each medal. The highest one, the Medal of Honor actually gives the holder certain privileges and is as close to sacred as these awards get. If other lives or an entire mission had been at risk, or if he had died in the process of rescuing others, you might see a Medal of Honor here.