There is no claim that the density means anything, so there isn't any need to adjust it for population. I imagine the title "OpenStreetMap node density map 2014" was somewhat carefully chosen, and it manages to say what is shown.
If you read the link that Vik1ng posted, the author seems to be fairly clear that they were just making a pretty picture (and quietly points out the problems with the projection).
Density here is number of nodes by area, and if you distort the area it stops being density. It's like comparing the weight of different objects on different planets. "My shoe on Earth weights twice as much as my sandal on Saturn" isn't very useful.
And thanks for mentioning the link, but I don't see where they say it was made for a pretty picture.
If you read the link that Vik1ng posted, the author seems to be fairly clear that they were just making a pretty picture (and quietly points out the problems with the projection).