From Sonic Retro
Magazine Preview #05 - Sonic on a Vine in Prototype Mystic Cave Zone
There is no vine in Mystic Cave in our prototype. All gates are opened by a switch whose graphics hadn't been properly implemented. In the left screenshot, there is no gate in sight, suggesting it's one of those vines in the final that move Sonic up or down, instead of opening a gate.
Don't make a big deal out of the "swinger" description. It's probably written by someone who wished to add more drama to the statement. There's no evidence the vine is supposed to swing.
We know the screenshot is not taken from the final, because it came with a bunch of other shots (#2 and #6) that were definitely from a prototype. In fact, we can deduce from the other pictures that this one came from a newer prototype.