The numbers you have found are the numbers averaged over all kind of projects (new projects, old projects). It is well known that the average daily LOC output in the industry is not high. Once a project has reached a certain complexity or maturity, people spend more and more time on debugging, refactoring, documenting, profiling etc.

For a new project, 1500 LOC within a weekend sound possible if the goal is to quickly produce some kind of proof of concept. You would probably spend the following days on redesigning the whole thing, though :)