You will have measured 2000 ml. I'm being facetious here, because you are measuring 2000 ml and what it will produce as a reading.

You will not get 200O ml as and output from the sensor. As Gyvate explained, the units are totally different and depend on surface area, which can be worked out to volume per surface area.

mm per meter squared, the area of the cone is no where near a meter squared, so you have to do some math for the actual area. Once you do that , also the volumetric portion, you will arrive at a calculation for volume to mm per meter squared, for the volume you dumped into the sensor.

Edit to maybe explain further: The sensor knows it doesn't have a 1 meter square cone, it does know the surface area it has, it can measure the volume of rain that past through the bucket. It then extrapolates the known volume, (based on mass to tip the bucket) , and then can build a number as if it collected that number as though it does have a 1 meter square cone.