Yes, it was clear to me that Darby would have been there to make the tackle.
By rule, since the whistle happened before the reception, it should have been a dead play and repeat the down. The 15 yard penalty on Rex, although technically justifiable, seemed pretty cheesy and desperate from the refs.
I still think the #1 takeaway from this game is that the Bills are still not quite good enough to beat the Pats, but the refs were pretty godawful in this one. Embarrassingly bad. I don't buy any conspiracy theories--I just think the refs were lost out there and botched this play badly.