Yup, you got it.
Rolling admissions means they fill the class as they get applicants they like, and so the later in the cycle it is, the fewer seats are left.
(By contrast, schools like HBS wait to get all their candidates and review everybody together, then select the ones they like the best.)
Being early is ALWAYS an advantage whenever a school has a rolling process.
If you're applying to the Evanston campus, then you just need to figure out when you want to go. Our assumption (not known as a fact, just a guess) is that they get a lot fewer applications for the January start, so it would likely be less competitive to go for that option. A strong candidate will typically find it's fairly easy to get an offer at any EMBA program as there is generally a lot less competition.
So, basically what you said.
EssaySnark