[1] Popular myth nature of this quip aside.
The fallacy pertains to people who argue that a causal chain will necessarily play out, or that a particularly implausible chain will play out with high probability.
The fallacy does not assert that slippery slopes don't exist, and the mere act of arguing that a plausible slippery slope has a nontrivial chance of occurring is certainly not the Slippery Slope Fallacy as it's formally defined. Saying that slippery slopes are inherently fallacious may be aptly labelled the "Slippery Slope Fallacy Fallacy".
Isn't this exactly what I described in that comment? I'm confused. Or maybe I was just not clear in my writing. Re-emphasized:
> not all things vaguely similar to previous paths to X result in X, obviously [=> even though some things do]
Also, I didn't really introduce slippery slope as a strict logical fallacy. I said they were making a poor argument and identified it as being of slippery slope form. They responded to that talking about logical fallacies so we went on that tangent, but really I just found their specific instance not credibly inevitable.
I think this applies more to wider generalizations as opposed to very specific instances due to the nature of probability and finity of resources invested in continued attempts, but it can be applied in either scenario. Think Murphy's Law.