Only people with access to this org can join its meetings

Tomas_SI had to ask Premier support and their answer was:
"As you want to communicate with the different organisations who is not a part of your domain that's why we would be require to enable this setting "Anonymous users can join a meeting".  And yes this is secure. And if there is the limitation on guest users tenant then those guest wont be able to join so... Because they have restricted there tenant for the same.

And yes for External access there are 3 options which can be set according to the needs of the organization :

• Off Completely 
• On except for blocked Domains: In this you can mentioned the domains which you need to block.
• On only for allowed domains: And in this you can mentioned those domains for which you want to communicate "