ICANN does not define SMTP, and the "relevant quote" from SSAC in the article footnotes mentions nothing about it, either.
In fact, RFC5321 makes explicit reference to the possibility of an email address using a TLD as the domain in section 2.3.5.
I mean.. you can use emoji domains right now. They work most places for email. The part I found didn't work so well is emoji usernames on emoji domains. That has poor deliverability.
ICANN, by the way, heavily discourages such domain names, even though it can't actually prohibit them: yes, RFC 5892 explicitly prohibits emoji code points in internationalized domain names but so what? If registrars allow (and many acually do allow) registration of such names that only means that they violate some RFC and they already violate quite a lot of them. Who cares! Just pay the money and we will delegate you whatever names you want.