In this case, where you're probing for user names or emails, you don't own the email, so you wouldn't receive the verification yourself, and thus wouldn't know

In this case, where you're probing for user names or emails, you don't own the email, so you wouldn't receive the verification yourself, and thus wouldn't know if the account exists. This is exactly why most password reset emails say "if you didn't request this, please let us know, as someone may be attempting to access your account".