[discostu]

If the ball bounces off the defender into the air it’s a tip ball and interference rules go away. And really we aren’t assuming the interception would succeed, we’re seeing that the ball was nose diving into the ground at least 10 feet in front of Gronk and he had been moving to the back line through the whole play.

[Steve_Seattle]

"If the ball bounces off the defender into the air it’s a tip ball and interference rules go away."

Gronkowski was held before the ball was intercepted; the rule you refer to applies after the ball is tipped by the defender.