The other possibility that we're not considering here (although this is pure guessing on my part) is that the person who was offered the $100 buy-up, was not being offered the buy-up under the new "if all elite are upgraded" program, but rather was being offered the buy-up based on his fare class (I think someone noted they're starting to put this on the website). There's a possibility that he was on a Y or B fare and non-elite and the cost difference for a first class fare was $100 and it was offering him that opportunity, in which case this seems legitimate.

Otherwise, given the OP was in the back of the bus and someone was given an upgrade opportunity over him is concerning that this program isn't "if all elite are upgraded" as they have said.

Of course, there's usually a plausible explanation, so hopefully we'll find out.