I’ve seen a photo from last October’s race weekend. I honestly can’t tell what kind of knot it is because the picture is taken so far away. Doesn’t look like a bowline to me. The knotted portion of the rope does look bigger than it would for a bowline. That said, I don’t know why anyone would tie a slip-knot for a rope-pull. That’s just stupid.
It’s just something to grab, it’s not like you’re tying down a canoe on top of your truck with it. Since you’re only putting about 2-lbs. of force on your garage door-release, I’m sure it works as well as anything else, including just having the loose end dangling.