In determining the meaning of any Act of Congress, unless the context indicates otherwise-- the words "person" and "whoever" include corporations, companies, associations, firms, partnerships, societies, and joint stock companies, as well as individuals;

But Dartmouth only interprets that corporations should get person rights in that particular case and perhaps by presumption generally. Article I Section I of the US Code does not determine the meaning of the word person in the Constitution (only in statutes) and even explicitly mentions that corporations will only be assumed to be persons in statutes "unless context suggests otherwise" (the ability of context to suggest otherwise actually implies Congress's belief that corporations are not necessarily persons under the Constitution).

I think a statute could limit corporate speech and we could still generally assume that the word "persons" in statutes (and perhaps the Constitution) includes corporations. Corporations don't need to have the exact same Constitutional rights as natural persons, and the word persons doesn't have to mean the same thing in the Constitution and in every statute. It could be presumed that corporation have all rights unless circumscribed by Congress. (But you wouldn't allow Congress to circumscribe rights for natural persons.)