Can't speak to what SSI requires as a bare minimum, but I know my SSI shop has an in-person class in addition to the e-learning module.
That said, I would be surprised if the requirements, in addition to the written test, don't also include demonstrating that you know how to analyze a cylinder and set your dive computer to the appropriate FO2.
Even if not required, I'd do it. There have been lots of posts over the last year by "nitrox certified" divers that demonstrate an alarming ignorance of the fundamentals. They read a few threads here, pick the parts that validate their unsafe practices, and then act like instant experts.
Nitrox is a very safe and a very useful tool. It isn't hard, but I wouldn't be in a hurry to skimp on the training. Often you'll learn much more than what is on the official curriculum, create a point of contact if you have questions, etc. Take advantage of that.