How does it detect up-ness?
Downdetector was indeed down during the cf outage, but I think the index page was still returning 200 (although I didn't check).
Running a headless browser to take a screenshot to check would probably get you blocked by cf...
It just fakes it as far as I can tell.
script.js calls `fetchStatus()`, which calls `generateMockStatus()` to get the statuses, which just makes up random response times: