Construct works offline, so it can load even if the web servers are not responding at all.
The account service is also hosted separately to the website. So if the website goes down, the account service can still respond and ensure people's logins work. (I've see this working correctly when the website has had brief outages in the past.)
If you clear your browser cache though, or you're trying to load Construct for the first time, and the website is down, it won't be able to load. Normally things come back soon! Recently we've had a couple of issues caused by the sheer volume of traffic the website has been dealing with, but hopefully we'll be on top of that soon.