(i.e. non beehaw members cannot post on beehaw, but beehaw members can go interact on other instances). But as far as I understand that’s not how it works.
It depends, you believe that's not how it works because you're thinking of both sides defederating each other, but defederation is one-side.
For example, beehaw defederated from lemmy.world but lemmy.world didn't defederate from beehaw, so lemmy.world people cannot participate on beehaw but beehaw can participate on lemmy.world.
It's actually a bit more complicated than that, since lemmy.world people can still participate in beehaw discussions but only lemmy.world people would see those comment, I think also other instances that are not defederated can but I'm not sure about this.