There is no such thing as an "authoritative dictionary", correct. Notice that I mentioned it would be best to "choose a specific authoritative dictionary". A project may choose a dictionary and label it as such, to avoid the never-ending arguments that are bound to happen as a result of everybody having a personal preference or the desire to argue non-productive points. The dinosaurs might have screwed up, just like the programming style guide authors that have elected the use of space indentation over tabs, but following the dinosaurs is better than seeking validation from the GitHub drunk every time an issue is raised.

The spelling of open source could depend upon the context, for example:

> out of date (three words, hyphens when attrib.)
>
> Her wardrobe must be out of date.
> An out-of-date passport.

__ __

> open air (two words, hyphen when attrib.)
> Getting out in the open air.
>
> An open-air swimming pool.

__ __

For open source (this is the rule Wikipedia follows):
> Open source is a philosophy.
> This is open-source software.

luke-jr mentioned this and is correct. However, given this is a collaborative effort, it may be difficult to explain the concept to everybody. Using one consistent spelling might be better than many people incorrectly using inconsistent spellings. I'm happy to change it to "open source" or edit based upon the context.