Censorship is the suppression of speech, public communication or other information
The argument is it isn't suppression because as a private company, people can still choose to engage in that speech elsewhere. The government can actually make laws to stop people from saying certain things in any context.
When a private company does it, they can only do it in the medium they control and for people who choose to use that medium.
It's simply incorrect to say that private companies cannot engage in censorship.
If they have enough control over a medium to effectively suppress speech, then yes. The argument is whether that is the case here. The definition of censorship is not merely "any case where someone prevents someone from saying something". The bar is higher than that.
The argument is it isn't suppression because as a private company, people can still choose to engage in that speech elsewhere. The government can actually make laws to stop people from saying certain things in any context.
When a private company does it, they can only do it in the medium they control and for people who choose to use that medium.