I recently watched DivKid's video on the Joranalogue Contour 1. At about 11:54 he shows how to use the Contour 1 as an envelope follower. Am I right in thinking that any slew limiter can serve as an envelope follower in this way?

Not really...if you compare it to the Maths, for example, you realize that the Contour 1 is a bit more like a Serge USG, so you can define the attack and release of the envelope follower function with it. But at the same time, it won't be an exact envelope follower, as this will tend to slow down the response to changes in amplitude, and a real envelope follower should track those changes a bit more "precisely". So while this might work for a sustained signal source, it won't work so well for something expressive...say, for example, if you wanted to play lead guitar through the synth, you'd WANT the exacting envelope follower output values so that you can continue to capture the guitar's true dynamics.