Yes, but even if the referenced security risk is patched you would still be able to find out if some has an account or not since a password reset page will tell you if it has successfully sent an email to an account.
They can tell the user to await an e-mail from them with the confirmation link. Then if the e-mail address is already in use, send an e-mail saying, "somebody, probably you, tried to register as <new-username> on <site> but we have you down as <old-username> already". Otherwise, send a normal confirmation link.