According to the article, the only part illegal about "simultaneously deplatforming" someone would be if the action were coordinated by competitors ("we'll ban John Smith if you do too"). I don't think there would be an argument with the same legal reasoning if platforms independently decided to deplatform someone, or even if they saw another platform deplatform someone and let that influence their decision.

The Reddit example might be closer to the type of coordination it targets, but it would be hard to apply anti-monopoly legislation to volunteer Reddit mods.