There is maybe a split second between the time the ball is touched and when Keuchly actually wraps up Gronk. Yes there was some contact there and it would normally be a PI, BUT the pass is undercut about 7 yards in front of them. Here's a gif of the play for reference:
When that happens, you employ this rule from your link:
Actions that do not constitute pass interference include but are not limited to:
(a) Incidental contact by a defender’s hands, arms, or body when both players are competing for the ball, or neither player is looking for the ball. If there is any question whether contact is incidental, the ruling shall be no interference.
(b) Inadvertent tangling of feet when both players are playing the ball or neither player is playing the ball.
(c) Contact that would normally be considered pass interference, but the pass is clearly uncatchable by the involved players.
(d) Laying a hand on a receiver that does not restrict the receiver in an attempt to make a play on the ball.
(e) Contact by a defender who has gained position on a receiver in an attempt to catch the ball.
Gronk was too far away from where the pass was intercepted to make a play on the ball or even the player for that matter. Therefore it is an uncatchable ball, and the flag gets picked up.