Yeah, I was wondering that myself.
I have never seen an external circuit to a house, such as a water or waste-water pump protected via an RCD.
Outside garden lighting where the lamps can be accessed yes, but something that is usually only accessible to competent persons, no.
Next thing being, this pump does not have a neutral (being 400V 3-Phase), so how would the RCD detect a leakage from Phase(s) to the required neutral?.
Even all the controls (Pressure switch, etc) are 400V.
I must say, I've never struck this problem before.
Same side of the coin, ever seen the price of a 3-phase RCD before?, they aren't cheap.
Let's face it, these days if you're not young, you're old - Red Green