If websites authenticate with username and password combo chosen by the user, then credential stuffing is neutralized if the user avoids re-using the same combo, effected by the user selecting at least one of a different password or the selection of a different username.
If instead of a username, an email address is required to register, that generally results in one less degree of freedom; rather than being able to create a username with Website B that differs from the username they created on Website A, absent the use of a wildcard/catch-all mailbox or forwarding service (which are not straightforward to set up, and almost nobody has one), the user is required to disclose an existing email address.
(It also increases the surface area for attacks, since the malicious website, now knowing the user's email address, can attempt credential stuffing with the user's email provider itself.)
You can balk at whether or not these are negligible differences, but it's non-zero. Therefore, all other things held equal, then strictly speaking it is more robust.
