Actually that would make sense if it was heat activated! The vacuum would prevent any convection/conduction so the metal would heat up to much higher temperatures with only EM radiation as a way to remove heat from the strip. That would make it way easier to heat the device up enough for thermal expansion to activate the relay.

Pretty cool if that's the case, I've never heard of a device like this I know some temperature gauges use this principle, but I've never seen a relay like that prior to now. It's possible that they left out the getter and it's just a low vacuum, which would work fine for something like this.