I think the difference comes down to prolonged exposure to lower levels (eg medium exposure) vs short-term exposure to high levels.

The canary is for the latter scenario.

If the EPA says it can kill "in minutes" [1], then I'd be inclined to believe them over "you'll feel it if you're exposed to it for hours".

1- https://www.epa.gov/indoor-air-quality-iaq/protect-your-fami...