Forward progress is defined as the player and ball moving forward. It doesn't specify that the player must be the one moving himself forward.

For example, lets say I'm on your 20 yard line, and you wrap me up for a tackle. You don't stop me, instead you pick me up, and fall backwards as you make the tackle. The ball is not spotted at the 20 where I stopped running, it's spotted where it is when my knee hits the ground.

The Bills players were pushing the pile forward. Therefore forward progress had not been stopped, even it if wasn't the ball carrier that was initiating the forward movement.