You shouldn’t need a device handler to get it connected to the hub, but you will most likely need one to get it to function properly. You should be able to get it to join the zigbee network, and it will show up as “Thing” to ST. In fact, it may have joined in this way already if you’ve tried to pair it; but the mobile app doesn’t always show you that something joined the network when it can’t identify the device. Check your Things tab in the app for something called “Thing”.
If it’s not there and you’ve followed the devices join procedure and pressed Connect New Device in the mobile app, then it’s possible the device can’t join a ZHA network. Usually ZLL devices are backward compatible with ZHA in some fashion, so it would surprise me if the device wouldn’t join the network.
Once you have it joined, then the fun begins trying to write a device handler for it. You could try the Hue Dimmer Switch code I wrote as it’s a battery operated ZLL switch, but you might have to write a new one.