Perhaps it might not be true? I suppose if you were reading the error then by definition it was down but places like Amazon's EC2 have to be careful about saying "EC2 is down for 10% of the customers in the east cost availability zone #2" or some such.

There are many moving pieces, hence the rise of sites like downforme.org

Thanks for mentioning that. I had been using isup.me but ran in issues when checking while some images failed to load on a site I volunteer for - it would check the responsiveness on the URL of the server itself, which gave a 404 as it wasn't supposed to be accessed by public and isup.me would interpret that as the server always being down... downforme.org seems to deal with that better, thanks!