I think * still means everything in this context. Specifically, it means including everything (every variables/columns) in each row selected. However, the count function doesn't focus on how many variables in each row, and instead it returns how many rows selected (how many airports in each state in this case). Accordingly, a row with all variables included is counted as 1 row; a row with only one variable is counted as 1 row, too. So you can replace "count(*)" with "count(elevation)" or count any other variable in your answer, and I believe it will give you the same answer.
By the way, I figure out another answer. You can also use count function instead of sum function in numerator; just replace "sum" with "count" and replace "0" with "null"; then same answer will return.