because if you know someone's email address you have now also discovered that they have an account with a particular service which you should not be able to do.
Can’t you do this anyway on the service? You are assuming there is no rate limiting and the passwords are used directly as bearer tokens to access the site.
But since you are giving advice to the DESIGNER of the site, why not simply tell them not to use passwords? https://qbix.com/blog for example
It's a tradeoff between usability and security, and each site should make their own decision about what is right for them.
It obviously makes attacks like the one in the article easier, but there are other ways to mitigate that.
An example often given for when revealing an email is registered would definitely be bad is dating website and pornography websites - where identifying someone is a member alone could be embarrassing or compromising.
Outside of such scenarios, websites may decide the increased conversion from a more streamlined registration process and lower numbers of support requests for login issues outweigh the marginal security gains from hiding that information.