You can make a DFF from NAND gates, though it's pretty obvious the simulator they wrote for the class can't perform that emulation. I was a little put off from the class when they tried to explain why they didn't want to cover them "because they were too complicated". It's pretty obvious to anyone who understands them that they're not significantly more complicated than the rest of the material. I gave up on the class right there, thinking the instructors and the rest of the class would be a lot of the same BS. I'm glad I got over it and completed the rest of the class as it was pretty fun.