I will say though, that wouldn't the government being forced to break apart major technology companies be a form of control in and of itself?

Yes. As John Locke would say, the very control that we enter into a social contract to engender.

Only problem is a) the government decides which rules are arbitrary and which are not, not the market

Correct. This is the fundamental basis of antitrust law. The argument is that the government has interests other than the profit and health of the company monopolizing.

b) now we have a bunch of medium-sized players, each with their own set of the exact same rules because they're all competing with eachother.

Also correct. Allow me to give you an example of how this works in practice:

If you earn less than $1945 monthly for a household of two in the state of Washington, you are eligible for Medicaid. The state pays for Medicaid. You do not pay a dime. The fee schedule paid out by the state of Washington is publicly listed. There are currently five major health insurers managing Medicaid for the state of Washington, but it's open to anyone. Provide health insurance, get the fees listed.

Now: you're poor and you wanna have a baby at my birth center. The state pays the same regardless of who you go with. You pay nothing. But me? I'm the business that fosters the competition here. See, you may come in having selected United Health to manage your Medicaid plan. You're going to talk to my receptionist and she's going to say "yeah, United's... okay? but they hassle you like hell on the following standard procedures we have to do, they tend to kick claims back twice as often as anybody else and their wait times are usually around twenty minutes. Molina, on the other hand, gives you a representative to call, rarely fights any of the stuff we do and oh hey, come to three scheduled visits before your due date and they'll give you a $100 gift card to buy a car seat."

These are not hypotheticals. no names have been changed. We jump through the hassle of getting people free car seats (it's a lot of paperwork) because we don't have to deal with United on the back end. So there it is. Market forces at work in managed care. The monopoly blown up into five (or more) competing factions, all working with the same size piece of the pie.

Part of the issue with freedom of speech is it will always be a nebulous concept.

Doesn't really matter tho because Doctorow's whole point is that poor application of anti-trust law creates de-facto free speech issues through the simple action of monopoly power. Fundamentally, the existence of monopoly allows the monopoly power to set whatever standards they want through market forces. The examples used are clear: eventually no one will be able to talk about politics because anywhere you're likely to talk about politics are under the control of a corporation that can exercise its free speech rights to control its environment. No animal carcasses necessary: if Google decides they'll only allow searches from people wearing clown shoes, we'll all end up with a pair of clown shoes by the end of the week.