Citizens United, as you mentioned. Read it -- it's clear that the logic is that corporations are collections of individuals and as such, they have the same speech protections as individuals themselves.

This is not the reason, actually. Laws can apply to different types of actors, it has been traditionally read that "natural persons" are flesh and blood human beings, while "persons" can include organizations, this interpretation is part of how companies can be legally held responsible for their actions and really _really_ regrettable and complex. It is possible that the 1st Amendment text was written prior to the distinction between "persons" and "natural persons" was clear, and that has been argued - but for the time being all "persons" are afforded free speech, including corporations.

No, the GP was correct. Read the Citizens United decision. The finding was based on the concept that the rights of the people who make up an organization extend to collective action taken by that organization.

The decision had nothing to do with corporations being considered legal persons or the definition of person.

I'm not saying that's not the logic of CU, but that is exactly the fallacy of composition.