In the context of this article, it is that security scanning software that companies/users are using seem to be indexing some of the 12-char links out of emails which ends up in some cases on public scan. Additionally, if domain.com/12-char-password is requested without https, even if there is a redirect, that initial request went over the wire unencrypted and therefore could be MITM, whereas with a login page, there are more ways to guarantee that the password submit would only ever happen over https.