Not sure exactly what you mean? Are you referring to a purchase flow where you are buying something and also given the option to checkout as guest or signin/signup?

I am only speaking to the typical signup flow that anyone can access signed out without putting in any information besides email/pw. If you are in a purchase flow where valid credit card info is already entered and is going to result in actual purchase $$, you've already excluded bots/hackers who would be trying to brute force account enumeration. It would be totally fine to confirm the existence of an account in a web form in this situation as it is not a flow that can be easily brute forced for free with little effort or info needed (like a credit card).