It is a population cap, you can read this proposal its like 5 lines of text.

If Swiss population growth were entirely attributable to the children of existing Swiss residents, then this initiative would be pointless because it wouldn't change anything, and we would not be having this conversation.

So yes, it absolutely is about immigration, regardless of the wording.

It's a migration cap. There's no provision on sterilizing Swiss women should the threshold be reached through birthrate…

No, it's an immigration restriction. There's no way it applies if the Swiss start having 5 kids each.

Apparently you are unable to understand that if people who have been crying about immigration for 20 years that now push this things does not mean they have changed their mind. They just try to hide what's obvious to confuse uniformed voters (like old people who just see big number and remember the 1970s).