This is more directly from WhatsApp, rather than from a third party.
WhatsApp FAQ – How to set up a payment method
"The payment methods available to you vary, depending on the country and currency of your ad account. Learn more about supported payments partners."
For India, the supported payments partners include "credit cards or co-branded debit cards", and "local manual payment methods, including Netbanking, Pay™ Wallet, and UPI."
Perhaps if you select a credit card as the method with which you want to make payments, you would be able to make those types of payments with WhatsApp Pay on an iPhone X. But the UPI option is definitely going to be subject to UPI system rules, which means, no UPI payments for you.