Verified answer

No, it's him. One, after he had his bypass surgery he jokingly wants to prove he's in great shape. Two, he likes entering from stage left, so if he's on the right side he'll run behind the curtain to the left side. The stage is smaller than it looks on TV, and he's pretty tall, so it only takes him a couple bounds. Three, it's a little joke with Biff. Sometimes you'll see Biff tap his watch like Dave's running late, so Dave "runs" behind the curtain but walks out, to show that it's his show and it starts when he wants it to.