Construct 3 works offline, and the way it does this is by always loading from the offline saved copy first. This means if the server is not reachable it can still start up just fine. It even means if you're online but the connection is super slow, it still starts instantly.
Since it starts with the last-used version, if it spots an update is available it tells you, so you can hit reload and start using it right away. You don't have to manually opt-in to updates; if you ignore it and come back later, it will update on your next visit. It just tells you so you can update sooner than that if you want.
(Beta releases use a slightly different update mechanism by the way - you have to opt-in to updates, but that's mainly because using a beta release requires visiting a URL that's locked to one version.)