How do you figure? I don't know a person who believes that Twitter endorses all content that is found on Twitter. How is it a speech issue in the other direction when Twitter is not saying anything?
Further, about broadcast media, the US still has the equal time rule [0] and used to have a fairness doctrine which required broadcasters to present both sides of a debate [1]. It's worth mentioning that when the FCC (all Reagan appointees) removed the fairness doctrine, Congress acted to enshrine it in law and it was vetoed by Regan.
[0]: https://en.wikipedia.org/wiki/Equal-time_rule [1]: https://en.wikipedia.org/wiki/FCC_fairness_doctrine
Banning someone or something is them saying something. Endorsement of the debate between different political views doesn’t mean allowing every idea to be acceptable. Letting say the anti vaccination people on your site directly means more disease risk to the general population. However, realizing that doesn’t require you to act. You might decide that’s acceptable where allowing say militant groups planning an insurrection aren’t.
In other words, voluntarily not saying something doesn’t give up your right to free speech.
IMO, forcing platforms to host content they disagree with is a free speech issue in the other direction.