Yeah, this. I bought Stellaris just after 2.2 came out, and the Scourge was my first end-game crisis. Fortunately they entered through my neighbor's territory and decimated him. I was unprepared but was able to defeat them once they spread out too thin.
But yes, this crisis is borked. Once they infest a planet, it seems impossible to evict them. I bombarded them from orbit with my entire fleet for two solid years, but the planet never went barren, and when I stopped the devastation level began to tick down from 100% as normal. I waited till well after 2600 to see if anything changed but finally decided to start a new game.
Anyway, bad luck they landed right on you from the outset.