Equal time is effectively defunct. Fairness doctrine was overturned in Meredith v FCC. The last remaining aspect of the Comms act of 1934 was the Zapple doctrine, covering explicitly candidates, and that was removed in 2014. There's no remaining regulation.
As for Section 230 protections for civil liability: Instead of suing the platform where the content was posted for slander, you'd sue the individual that posted it. S230 protections do not extend to where the platform's own speech (ex, Twitter's blog), and they don't shield the actual speaker, where you can still recover damages. Your own example goes to this end - Twitter didn't publish anything, CNN did, and he was able to convince a court that it was slander, so he was able to recover from CNN.
As for Section 230 protections for civil liability: Instead of suing the platform where the content was posted for slander, you'd sue the individual that posted it. S230 protections do not extend to where the platform's own speech (ex, Twitter's blog), and they don't shield the actual speaker, where you can still recover damages. Your own example goes to this end - Twitter didn't publish anything, CNN did, and he was able to convince a court that it was slander, so he was able to recover from CNN.