You could call the respective Admissions offices of both schools to clarify this. Based on what I read from the websites, I conclude the following.
1) Per http://www.law.upenn.edu/prospective/jd/apply/, early decision results will be sent by December 31. This does not mean that the results will be sent on or after December 31. That is, decisions could be made prior to December 31 - which would be in accordance with the quotation you present below. You are correct that the Round 1 decision release date for Wharton is anticipated to be December 20th. There is no discrepancy, here.
2) Where I believe the significance of Early Decision lies is in its binding nature. Note that, in order to participate in the joint degree program, you must be accepted by each school (law and business). If for some reason Wharton denies you admission but you apply to Penn Law as an early decision applicant, my understanding is that you would still be bound to attend Penn Law because you chose the early decision path. Does this make sense?
It's worth reaching out to make sure that this is indeed what is intended, but that's what a careful, plain reading suggests to me.
Hope this helps.