You’re pointing out a contradiction that isn’t present: the argument against compelled speech does not rest on corporate personhood.

In other words: there is no investment in a corporation having rights added to it as if it were a legal person. The sole interest is that the people within the corporation do not have their rights subtracted by virtue of how they choose to organize.

You can see this in how people actually talk about moderation online: it simply doesn’t matter if it’s a corporation, nonprofit, union, or individual running a service.