I think you are understanding it right. You need to be a Premium user in order to key share with others. You can be using the Free version to open a key shared files.
So the sharing person needs Premium, the sharee does not.
Currently, we do not support adding other passwords to a file, but it’s certainly technically feasible. The reason we still might not implement this is because we don’t think it’s a good way to work, and we’re hoping that if you have the need to share encrypted files with others, you’ll find the convenience of the Key Sharing feature to be well worth the small cost of Premium.