Slippery slope is only a fallacy when you have very limited, often a single, data point. You could say using the Emergencies Act is only a single data point or you could say giving in to the authoritarian nature of power hungry politicians has endless data points. If one sees the Emergencies Act as a subset of power hungry politicians, there are already more than enough data points to see where the slope leads. Even before the pandemic there were already plenty of data points to establish that PM Trudeau and his cabinet are power hungry authoritarians. It pretty much comes with the position.