"Against the Swiss constitution" doesn't really make sense here. This is a popular initiative; if accepted, it amends the Swiss constitution. Here's the text: https://www.bk.admin.ch/ch/d/pore/vi/vis555t.html

The only way you could argue an initiative is "against the Swiss constitution" in my opinion would be if it runs afoul of the rules: https://www.fedlex.admin.ch/eli/cc/1999/404/en#tit_4/chap_2

Unless you want to argue that this violates the mandatory provisions of international law, I don't think you have an argument. The text of the amendment specifically clarifies that any of the actions it mandates on parliament have to adhere to the mandatory provisions of international law, so I don't think that's an avenue you can pursue.