They are implying the great filter is actually the current step (advancing towards colonization explosion). The next step would be colonization explosion (aka colonizing space or other planets), but that could be cut short due to civilization eradicating itself. This is thought to happen either via climate change, nuclear war, or artificial intelligence wiping us out.

I personally am not a fan of the AI destruction theory because I'd expect AI to replace us and expand into the solar system. If it got to the point where it could kill all organic life, then I'd bet it's unlikely for AI to die off with us as well. Just my opinion though...