I understand the "explanation" such that the load has to be connected between L1 and S1. You can verify that by connecting a lamp to these two terminals and have the Zigbee receiver switch on and off. The seller stated that L is connected to L1 which means there is an internal connection. Therefore no need for an external connection.
The seller states also that S1 is switched. Given that L is connected to L1, this must mean that N is switched to S1. Not the best idea to switch N but will work. Usually one switches L and has N connected (wen using single pole switches).
As L is permanently connected to L1, there's no use in switching L with the double pole switch.
The resulting circuit would look like this:
The danger being that the heater's one side is permanently connected to L! I don't know if your local regulations allow such a setup.
As the Zigbee receiver is insulated, you can possibly swap L and N at the input to the receiver:
In this setup L is the switched connection and the heater would be permanently connected to N. Imho much better but the heater is switched single pole only. This setup should be verified by someone familiar with the electric ode where you live.
This drawing is based on assumptions from the meager "explanation" by the seller. You should at the minimum verify the assumption about the switching of N and the connection of L to L1 by the lamp test I described initially.
Better yet have the schematic approved by the seller or an electrician familiar with the electric code where you live.
My sincerest recommendation: Have the installation done by a qualified electrician to be on the safe side.