As you have only been introduced to elementary math(s), you may not be aware that median is also an average. Half of programmers *are* below average using one of the three usual rivals for 'average'. It would be hard to use mean, as it's not clear that you could find a rating system for which using mean would make sense.
Of course, it's not clear that there is any sensible numeric rating scale for programmers, and it would be interesting to argue about what the modal average would be, but it's just a throwaway comment which doesn't need to be analysed in such depth. The meaning and sense of the comment is clear.
If you are going to play the role of overly pedantic nerd, at least do it correctly so you don't get laughed at.