If it's the plastic moulded rim strip I'm thinking of it doesn't need to seal to the rim - the tyre and the rim strip form a sealed unit, where the only possible means of escape is the valve hole. If there's sealant underneath the rim strip (assuming it's the moulded plastic type) then the only possible point where the sealant could escape is the valve hole.
If it's the plastic moulded rim strip I'm thinking of it doesn't need to seal to the rim - the tyre and the rim strip form a sealed unit, where the only possible means of escape is the valve hole. If there's sealant underneath the rim strip (assuming it's the moulded plastic type) then the only possible point where the sealant could escape is the valve hole.