"Slippery slope" is a logical fallacy.
The antivax cowards had many peaceful protests previously without issue. They weren't getting their demands met because their demands were idiotic.
So they escalated to disrupting the functioning of the government. Using psyops tactics against civilians. Harrassing civilians. Disrupting emergency services.
And for what? It wasn't to increase awareness of covid restrictions. These restrictions were placed on the entire population, we were all aware of them. No it was an attempt to affect a change using extortion. Changes contrary to the democratic will of the country.
Since you love the slippery slope fallacies, consider the slope in the other direction. If an organized crime outfit used intimidation tactics to get their way, could they declare it as a "protest" and get off scot free? Where do you draw the line in that direction?