Its like Ferrari has a second team... Merc does not have a second team. They supply engine and bits to other teams, like Ferrari does to hass, and Alpine used to do. RB right out owns two teams.
Mercedes and Ferrari supply engines for most of the teams which leads to close partner relationships between certain teams, but nobody else has an explicit fully-controlled junior team like RB. And I agree they should have to give it up if they want to limit entries.