In a workshop setting we test cat converter blockage on a regular basis, we follow the recommended old guidelines from Corning (from the 70s) to test for pressure differential across the two combs
A max pressure differential of 1-2 psi on a 4 cyl modern engine at idle to fast idle is checked, anything above this means the converter needs to be replaced, please note the replaced word.
Sometimes we encoutner poisoned converters like silicon coated faults or coolant ingress or sometimes the engine ran too lean for a very very long time and burnt the blanket around it causing exhaust bypass, in this case the pressure reading is A OK but there is almost negligible temperature difference between the inlet and oulet (converter not starting the thermal reactor) in this case the converter is replaced too.
Washing the converter with water actually damages the fire blanket around the cores.
In short, thank you for also endorsing not to fall for these scam practices.