You have to have some kind of angle to see it. Looks like one of the camera shots might show proof one way or the other. I assume the referee gets to see every camera when he does the replay (which the audience often doesn't get)?
It's a lot like this play with Russell Wilson:
https://www.nbcsports.com/philadelphia/the700level/nfl-rule-says-russell-wilsons-lateral-illegal-forward-pass
IMO, neither play would have had conclusive enough evidence to overturn. The issue isn't that the players are moving, it's that the cameras themselves are moving (that's how the camera is able to follow a player down the field on these plays), so while we think it was thrown here, or it was caught there, it's impossible to know based on that. In both Wilson's case and Troy Brown's, it appears pretty clear they are releasing the ball backwards from where they are. I suppose due to them running downfield, the ball went backwards, but the forward momentum of the thrower, with the forward momentum of the Wilson/Brown made the ball end up further downfield making it illegal.
But I'll ask this, what if the person that caught the lateral didn't catch it? Do folks really think the correct call would be a 5 yard penalty and the Pats/Seahawks retain possession, as opposed to a fumble? If Troy Brown makes that "pass" and it's not caught and it's recovered by Pitt, would BB have called for a review and said it wasn't a fumble, because it was an illegal forward pass?
Maybe by definition and by the angle on the cameras, these were both illegal forward passes, but the spirit of the rule lived, IMO.