On a serious note: The general question, " is LMS X is better than Moodle" has been answered by Colin. (My +10 to it.) Your question now is, "I am going to give a MOOC. Is Moodle a good choice?"
I don't know. My only exposure to MOOCs is as a _participant_ in only a few of them. I think the only one which was running Moodle was the first Moodle MOOC "Teaching with Moodle: An Introduction". The follow-up is here: https://moodle.org/mod/forum/discuss.php?d=240593. But that was 2013. Things must have changed. Here is a report from Fall 2017: (found though a web-search, haven't watched it).
About the numbers: You can register 2 million users in any 10 year old machine. The question is how active they will be. There is a specialized forum for such matters: https://moodle.org/mod/forum/view.php?id=596. At least equally important is the lesson plan of the course, and the teaching resources. How exactly, you the teacher and the tools provided by the LMS, are going to teach for example 10,000 students. The former, hardware scaling, I would love to tackle, if it is Moodle. (See my profile. ;) The latter, the teaching part, sorry, never done and not too keen on doing.