The "five to ten minutes to teach" line from Margaret makes sound like it could be a lighter version of Cortex+ (which, in honesty, is already pretty light). This makes sense, because if they're planning on republishing the rules with every event, you don't want it to take up too much territory.
So, you buy the essential edition of the version of the Marvel Universe you want to play, which will likely have stats and backgrounds for the heroes and villains, and background of the setting, of the event it covers. Then, if you want to play an adventure path style of game through that event, you get the supplemental adventures, as well. If you just want to run your own game during that timeline, you don't.
The line that confused me a bit is, "...each with a premium and essential edition..." Is that market speak (like New and Improved) or are "premium" and "essential" different editions? Might they publish one book with the rules and one without if you already have them? I can't see that, unless it was a pdf/pod option only.