If you have it all in the same automation, it just won’t run.
But if you set up a separate smart lighting automation (From the three horizontal line icon in the upper left, not from the “add an automation” + in the upper right) just for the siren, you would be able to turn off the siren that way as long as the siren also around locally.
The problem is that since you didn’t change the security.mode, it’s quite likely that it would keep triggering and start sounding again and again and again every time the trigger condition was met.
This has happened to community members in the past. For example, if your siren is being triggered by a motion sensor, then every time that motion sensor detects motion the siren will sound again.
Or if it was triggered by the front door being opened, then every time you open the front door the siren will sound again.
This is why you will see discussions about taking the batteries out of the sensors or taking the siren off of power. It’s not enough to be able to turn it off locally. Since you can’t change the automation which is triggering the siren to sound in the first place, then that just keeps happening.
Plus remember that the app doesn’t work when the Internet is out, either, so you have to have tied it to a physical device which also runs locally.
This is why people are always asking if there’s a way to change the security.mode when the Internet is out, but unfortunately, as of this writing there just isn’t.