For the mini switch to work with Siri it has to be recognized in HomeKit. The new switches can show up in HomeKit before they show up in the Wemo app during the install process . But then will appear in the Wemo app once it is restarted. I have a Wemo bridge and about 16 Wemo switches, two dimmers and 15 different Wemo plugs. I am also on my third Wemo bridge. It is very annoying that the Wemo devices and bridge are so flakey - they are just fine in their own Wemo app but seem to come and go in HomeKit. I had a large collection before HomeKit was made available so I didn’t want to replace.
Yes I know how to use a web browser to visit the bridge - clear and reset it, as well as how to reset the plugs and switches. I wish there was real stability