The blind spot sensor, and cross path detection, is located in the rear tail light. For it to keep working, as it is unlikely they will use the existing tail Light, they need to remove the sensor and add it to the new deck lights, properly aimed. The sensor is built so you can take it out and replace it. I see no reason why someone who knows what they are doing could not relocate it but I have not personally tried to do that.
If you ask them exactly how they do it, you will pretty quickly figure out if they know what they are talking about, or not.
P.S. Keep your old tail lights to resell, they are worth quite a bit, as is a new box and tailgate.