Isn't it kind of obvious what Elseworlds means? When DC put out the first comic with that name, I don't think they had to explain. It's a simple portmanteau word. You take elsewhere. You take take worlds. There you got elseworlds. It's not rocket science.
But the more important thing is that if they put a label--really any label would do--in front of the movie in the advertising it's like a blinking amber sign that tells people that this is something else. And then they don't have to do as much dancing around to explain to people that it's outside the regular continuity.
Also people have these things called smart phones. I don't, but other people do and I understand they use them to control their lives. So all you gotta do is enter elseworlds in your phone or even just take a picture of the logo and the phone will tell you what it is. Crazy.