DND silences all alerts not just calls. Secondly, your method means DND is no longer available for any other use, such as overnight or in meetings when you only want calls from a limited number of people to come through.huh? big whoop, this is not new, lol,
I have already been doing this in iOS 12 using Do Not Disturb.
So it is new.
Until they add each other into their contacts, or the "Siri intelligence" allows calls from people in your recent-calls list, as you suggest. That's a brilliant idea.... if two people unknown to each other have this feature enabled, they will play phone tag forever.