Full-stack just means they do back-end and front-end work. Don't try to make it more complicated than that.

The reason I put 'full-stack' on my resume is because I know a lot about back-end programming AND front-end programming and don't want to be in a role where I am only supposed to contribute to one of them. Because the majority of web development jobs are either labelled as 'front-end' or 'back-end'.