they can’t link the account
Google can, since it's their OS and their payments.
But even if you're not using Play Store to pay, the vast majority of people who are paying are going to be using the same device/IP/fingerprint/etc. where Signal is running, so it's not impossible to put the two together to link an account to a specific payment, if say, a government forced them to do so in order to track someone. It's just a bad idea for several different privacy reasons.