The document you link to should still very much apply to MorphOS 3.17, even though it was last updated in 2013. Nothing has changed that would break any older software or software development methods, but it obviously won't be able to cover any newer stuff added to MorphOS since there.

There's also some developer documentation right here at MorphZone. If you click the 'library' button at the top of the website, there's some pages dedicated to getting started with software development on MorphOS. Scroll down a bit or search for 'development' on that page.