I believe they were rude, or at least thoughtless. Why did Mary and Tess decide to put together this dinner in the first place? And why with the same set of friends/guests? Was it because they thought they'd go hungry at Lucy's event? So they thought it'd be a good idea to eat before they went? And then they thought they'd invite the same crowd?
I have no idea if this is the case, but just to offer one possible answer: out of the 20 people invited, I can easily see 7 or so of them being a kind of "sub-group". Meaning, they're not super close with the host (although they are obviously friends, since they were invited to the party), and may in fact be closer with each other.
So, Sub-group wants to meet up earlier to hang out and have a sit-down dinner somewhere, and then they all plan to go over to the Big Group party when dinner is over. Just in my social circle, this would be totally reasonable, even if they did end up getting to the bar quite late. In a group of 20 friends (IMHO), there's always some dividing into groups. I see the 13 people at the bar the whole night as being the closer friends, the core group of the party who are close to the host, and then the 7 late comers as the supplemental group, so to speak, who may be less close.
I wouldn't do this if it conflicted *that* much with the planned party, of course, but I can see why/how the dinner may have just ended up organically happening instead of being sabotage.