The article addresses this: what happened with .su is part of what caused ICANN/IANA to update their policies to not have defunct country codes stick around.
Additionally (and more importantly), SU is still reserved in the ISO 3166 list (https://www.iso.org/obp/ui/#iso:code:3166:SU), so if United Kingdom somehow convinced ISO 3166 MA to reserve IO then it will be messy.
