If I invent a machine that passes out six thousand flyers per second, and allow people to feed text into it at will, then yes, it is insane to describe these as my "speech". I may provide a mechanism for the words to get onto a page, but I have zero agency in the process of thinking them up, drafting them, and enacting their distribution. I am providing a
mechanism for
others to say things.
However, you would probably say I start to become liable if I erect a giant wall on which all of these flyers are posted, and allow illegal content to remain hanging there even when I'm informed of it and aware it's illegal. This gray area is exactly what Section 230 is designed around.
It's totally unreasonable to expect a platform operator to act as the speaker of a post the moment it is posted. But after becoming aware of a post and the reasons it may be objectionable, they start to gain a sort of post-hoc liability.
If someone prints out child porn, glues it to a yard sign, and plants it on your front lawn, you should probably not be liable for arrest starting that instant. But if, after coming and going and seeing it over and over for a week, you make no efforts to remove it or report it, then you should probably be liable for arrest. This is exactly the logic behind Section 230.