Their description doesn't really match what they do:

"Observatory by Mozilla is a project designed to help developers, system administrators, and security professionals configure their sites safely and securely. "

All they check is if you have a few security headers and consider that "secure".

There is a LOT more to website security than adding a few extra headers and HTTPS to your site. Even if you get an A, it doesn't mean anything.

To give an example, Google gets a D, CloudFlare a D, Youtube a C+, etc..