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.
"Governments, private organizations and individuals may engage in censorship"
It's simply incorrect to say that private companies cannot engage in censorship.
Most comments and up/down votes are about what people like/dislike not about what's either right, interesting or insightful.
It is what it is, and the mods couldn't care less.
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.
Doesn't make it bad, necessarily, but it is censorship.