I've searched out and read close to 50 Maine Coon breeder contracts online, mainly from the US and not one of them did not state something like "indoor or outside on a leash". You can take anything I say anyway you wish, but the facts are what they are. Granted there are many more breaders that 50, but when that line shows up in every one of them.
I don't see how a number of 1% risk can even be attained, sounds like an arbitrarily picked digit. There are plenty of things that can keep cats happy and healthy inside.