The plumber’s idea is to have the water that used to go through the radiator go through the 1/4-inch tubing. The water will go through it if there is no air block. The hot water will then warm the room according to how many square feet of floor there are. Each square foot is worth about 30 btuh, so a 5 by 6 exposed floor area can give about 900 btuh which is equal to 1-1/2 feet of baseboard radiator. If you needed more than that, then the room could be cold.
Feel the floor and if it feels cold, then the water is not getting through the tubing. If the floor feels neither hot nor cold, then the water is getting through as you can sense a temperature of 90F as no change from your body. If you are still cold in the bathroom, you need additional heat.