For baby 10M+1 are they going to tell a Swiss woman that she can’t have a baby?
In any case, this is definitely not conformant to the treaty that the Swiss have with the EU around rights of free movement.
It's primarily an attempt to control immigration.
At 9.5M, the government has to start limiting the issuance of residence permits and start renegotiating international commitments that drive population growth.
At 10M, the government has to terminate the free movement agreement with the EU.
The right-leaning parties bring up something like this every few years. They always get shot down.
This wouldn't happen because it's not actually a population control measure, it's an immigration control measure - when the population gets above 9.5M, Switzerland would start shutting down immigration/asylum. There's nothing in the initiative that would set controls on births by Swiss citizens. (and it would be unlikely to be needed since Switzerland is facing the same low birth rate of other western countries)
Organic population growth doesn’t have to be criminalized or authoritarian-controlled like China tried.