Ask siri to set alarm silently?
Settings > Siri &amp; Search > Voice Feedback

Your choices are :-


Always On
Control with Ring Switch
Hands-Free Only


So really to prevent her answering, you'd need the 2nd option &amp; the ring switch off.
The info in the pref itself says this will only work if you don't use Hey Siri - you will have to use the double-click to wake her.

The alternative is to turn the ringer right down, Siri volume is tied to that. I haven't tested how that will affect an alarm though. It might be somewhat self-defeating.