"Imminent lawless action" is not the same as "shouting some words". People shouted "fire" in theaters hundreds of times - e.g. during plays - and nothing happened. Only when it causes - or is imminent to cause - the actual action, can it be punished. Just as saying "shoot him" is not restricted speech - unless you are saying it to an accomplice who then murders a person. There needs to be an independent lawless act, and speech needs to be used as a prerequisite or a tool for it.

> Private corporations can deplatform whatever they like for any legal reason.

Legally, they can. Morally, they shouldn't.