The problem with "audit" mode is that it is more than just not getting a credential (which I like most people who already have their educations don't need) but that often you can't take the online exams either. I still want to know if I've learned the material properly!

Sure they could do that. You're essentially asking them to implement an additional feature to handle a new use case—a totally reasonable thing to ask.

But it seems like it's really a stretch to say "it's a dark pattern to not implement this feature that covers my use case". If not implementing a desired feature is a "dark pattern", then I'm not really sure I know what constitutes a dark pattern

The behavior I'm describing is how the website used to work. The put in extra work to prevent users like me from using it this way.