It's absolutely true that the Left uses the Slippery Slope fallacy as a "Fallacy fallacy".

Truth is irrelevant to validity. What the Left does is claim a Slippery Slope fallacy to say that because it's a fallacy, the prediction will be false.

They know that what they are doing is a 1:1 sequence of events to acculturate people to the next step in the process, and they need a rhetorical weapon to summarily dismiss any identification of their actions.

A 1:1 sequence of events is not a fallacy, it's implication. The Slipperly Slope fallacy exists because you don't have a 1:1 implication. That's why I compare it to catastrophization.

The point is that the Left knows what they are doing is a 1:1 implication, they're just lying. And, you should always meet a claim of Slippery Slope with "Fallacy Fallacy" when you see it (because that really is what it is); and you can always explain their own objectives to them, which they like even less.

Some fallacies get bastardized in this way for political purposes. The No True Scottsman is another one.