Is it really possible for a train to be already full months in advance? Especially given that it appears as if it was never open for booking in the first place…
On Trenitalia the train is marked as temporarily unavailable. So I think this is not a case of the train being booked out, but a case of the train not yet available (probably because there are planned engineering works on that Sunday) and the SBB booking system still being unable to provide useful error messages.