I think most people would make sure to say goodbye before parting ways. But I can also see confusion easily taking over, especially if the dispersing party is chaotic.
At our family get-togethers, "goodbye" is the most chaotic time, and it usually isn't a cut-and-dried "goodbye-I-love-you-kisses-and-hugs," but a lingering, mixing, yakking, edging movement of an entire group toward the door. A lot of times, our kids will decide to go with another family member at the last minute, but the group may hang out for a few minutes after permission...and then sometimes the kids or the adults change their minds, etc., etc..
I'm not trying to stretch for an excuse for this family -- It's just easy for me to envision chaos and confusion leading to a misunderstanding.
While I can see, to an extent, what you're saying, I will have to agree to disagree. I just don't see that in this case and have to stretch my brain a little to far to wrap it around that idea.