No apologies required! If we see this question come up a lot we can add it to the FAQ.
MongoDB does not release the stack for Atlas, our SaaS. That's possible because we own the copyright to the source code -- we don't have to issue the software to ourselves.
The blog post we published announcing the change covers this, as well as our motivations and expectations in a lot of detail:
https://www.mongodb.com/blog/post/mongodb-now-released-under...