I don't believe that poison is actually an effect, only a special condition. While in the past special conditions could only be produced as a result of an effect, we already know that ink is not an effect yet is still applied, indicating that this is likely no longer a requirement.
Additionally, moves that are known to inflict poison also have other effects. For Poisoned Smash, several smash attacks that necessarily have other effects (e.g. ZSS's down smash, Villager's down smash), so I doubt it replaces the effect on the smash attack. Likewise, Yuri Kozukata inflicts both poison and paralysis simultaneously. --SnorlaxMonster 00:38, September 8, 2019 (EDT)