Yeah, I've always found it "curious" (as in, bullshit) that Slippery Slope arguments are invalid, but arguments that invoke the Overton Window are perfectly fine.

They're the same concept. And that concept is also in physics. "Things in motion tend to stay in motion unless acted on by an external force".

Not really. "Slippery Slope"-type arguments are fallacious when there is no evidence provided to support the slope. The fallacy is arguing that the closeness of each "step" in the slope necessarily means that these steps will likely happen, but not providing evidence to support that these steps will occur. If one provides evidence which support that certain steps will likely occur, then it's not a fallacy.

As an aside, there is something called the "fallacy fallacy" wherein pointing out a fallacy in someone else's argument doesn't necessarily mean that the argument's conclusion is false.

With that said, I'm not making an assessment of the above poster's reasoning.