Population Control ≠ Immigration Control.
Saying that you’re going to cap your population necessarily implies you’re going to take policy measures to grow no further than the capped amount, which are by their nature, repressive.
That said I did see in some other comments earlier after I posted this that this is a back door way to axe the Swiss-EU bilateral agreements all at once. I don’t know how true that is, but if that’s the goal, Switzerland doesn’t need to take such a back door approach. Just put it on the ballot like everything else.
It was put on ballot as such, and overwhelmingly (by Swiss standards) rejected. Now it's time for trying different backdoors (not the first try either). It's a constant circus, EU and the EU citizens being the demon eating at the Swiss well-being.