Slippery slope fallacy = "These only vaguely connected things will lead to one another."
Pipeline = "This community has considerable overlap with another community, and participation in one is likely to lead to participation in the other."
In other words:
Slippery slope = when used by people I don't like
Pipeline = for me to use against someone I don't like
A lot of people seem to not understand what a fallacy is. They think that if something is fallacious then it is necessarily false, which ironically is a fallacy in and of itself (the fallacy fallacy). All that a fallacy is is an argument that does not logically follow from the starting axioms and the conclusion. Slippery slopes are predictions, sometimes accurate ones, but it does not necessarily follow that some of a thing leads to more of that thing, thus it is a fallacy.
Well, that's the thing, though. It's only a fallacy if it doesn't follow logically.
Not that it matters. Observed trends, such as radicalization pipelines, are not logical arguments, they're... observed trends. And suggesting that noticing those trends is actually engaging in a logical fallacy, as we see on display here, is just some bad faith cargo-cult no-you-ing.