It implies that the job sits within the editorial structure of the publication. You are looking at how the publication can use social media for editorial purposes. It distinguishes it from a purely marketing position, where you would probably be sitting with an entirely different set of colleagues with different purposes.

Editors often do strategic work. They are not always sitting and literally commissioning and editing articles.

I think a social media editor needs to use social media to keep abreast with trends. But tweeting on behalf of a brand is only a very narrow part of the role.

And there are plenty of photo editors who don’t take photos in their current position. The point is that they know what makes a good photo.