The first amendment right to free speech doesn't compel any company (whether publicly traded stock or privately owned) to give you a free publishing platform.
Did you even read the article? It clearly notes that the First Amendment doesn't apply to private companies. The article is about censorship, not about the First Amendment.
The article doesn't claim that what Facebook and Twitter did was illegal. You're conflating whether they had the legal right to censor, with if they should have used it.
Not true. Telephone utilities are generally publicly traded but it would be a free speech issue if they were to censor on their platforms.