I saw Beckham do this on MNF against the Cardinals and was lauded by the announcers for his football smarts.Didn't watch the game, but is this talking about a player being forced backwards by contact or willingly retreating?
Forward progress only applies when a player is stopped by the opposition and forced backwards. If a player catches the ball and then willingly/intentionally runs laterally or backwards and goes OOB then forward progress doesn't apply and the clock should stop as the play is over due to going OOB.
If a player is stopped in bounds by the opposition and then forced out of bound laterally or backwards by the contact, then indeed the clock does not stop because the play was "over" in bounds.