There's this running idea throughout the hour of people who appear to have everything but actually have nothing. Obviously, Betty sees Don as a winner when we know (and Francine can tell) how utterly lost he's been since the divorce. This is an episode in which Don slowly begins to reclaim his mojo, telling himself, "I want to wake up. I don't want to be that man." He starts the episode unable to swim a lap without coughing, and ends it outracing the younger guy next to him for half a lap, and in between has a pair of mostly successful dates with both Bethany and Dr. Faye. And yet the only time he seems truly happy (and that includes what Bethany does in the back of the cab) is when he's holding Gene in his arms at the birthday party (and after Betty has happily brought the boy to his daddy without the usual drama that comes between them).
I could be wrong, but I think the kid Don beats in the pool is Joe, the freelancer that Peggy fired. And as Peggy said to him, Don doesn't even know who you are - it looks like Joe glares at Don as he celebrates the victory in the pool, while Don looks bewilderedly at Joe trying to place who he is.
Betty's friend Francine tells her in the kitchen to be careful when Don comes to the party, because Don has nothing to lose, and you have everything. "Don has nothing to lose" could mean that she thinks he has nothing, or it could mean that he has nothing to lose by upsetting his ex-wife and her new husband because he's so confident and doesn't care. And the similarly ambiguous "you have everything" could mean that you do, in fact, currently have everything, or that you have everything to lose in a confrontation with Don - your new husband, your relationship with your ex, and your . Later, in the final scene of the episode, when Don is holding up Gene, Betty says, "we have everything" while starting off at a rejuvenated Don holding Gene up and smiling. And it begs the question, who is we, and do you mean you have everything, or that you have everything to lose? Or is this just Betty acting like everything's perfect again, like she did from the start of the show?