[Citation Needed].
Straight up, a singular company having control over something was well established at the time of the First Amendment.
1. "Monopoly" is a greek word which was used to describe a company that had a monopoly in Greece. This long pre-dates the first amendment.
2. Kings routinely gave out exclusivity of a product to a company, this is the whole backstory on the Tea Act [1].
3. Being denied access to something isn't a new concept. They could've written the first 10 amendments to address private citizens and choose not to. It's not like private citizens didn't try to surpress speech.
Times are not new. Technology is new. Every problem is old.
The few hundred corporations that existed by 1800 were chartered by individual states, couldn't do business outside that state, weren't allowed to participate in the political process, and couldn't buy stock in other corporations.
During and after the civil war is when "modern" corporations started to show. They made huge profits from procurement contracts and took advantage of the disorder and corruption of the times to buy legislatures, judges and even presidents. They lobbied and bribed their way to more relaxed laws that allowed them to accrue even more money and political power, leading to the first Gilded Age.
Then in 1886, the Supreme Court decided that Corporations are People, too[1], and it went downhill from there (except for a brief blip in the 1930s).
[1]: https://en.wikipedia.org/wiki/Santa_Clara_County_v._Southern....