I am not an expert, but I have been using the UPI ever since it's available and I got my refund back using this method as well as the alternative which is the traditional way of contacting the bank. This site support system is provided by the government to minimize the workload and encourage the users to report even if there is scam or fund sent as mistake.

We upload the UPI payment proof along with TXid aka ref no and a little description then our bank account and whether the TX is p2p or P to merchant and this works for both the same but I did for p2p though. Once we uploaded there will be support Id or some sort of ref num will be given and then the process will begin which is intimating our bank side and let them know the deposited bank side the funds deposited as mistake so the other party's bank will contact the respective person and see if it's that person agrees yes it is not supposed to be credited then the bank will reverse the funds and this only works if the person accept the fund deposit is a mistake and they give consent to the bank to reverse it.

So whether it is a peron or merchant as long as they gave UPI there will be someone who is real or alteast their identity is being used by the bank account and their details can be fetched by bank in no time if necessary.