If they truly believed that your pellet had hit someone, they would have taken your gun there and then, as that is a proper criminal offence. What a load of . They make up these blatant lies to get a response. As you say, at 20m, they'd probably be in hospital, not whinging about the noise.
As far as the noise complaint, unless you have your target attached to a metal bucket, that is also rubbish. As you know persistent noise complaints are investigated by some council jobsworth with a microphone, and I doubt he would have much to record.
Just make sure you don't stop doing what you are leg