In that case, the bill would have no effect, since suspending Section 230 protections only has an effect if there is some other basis for liability, and if misinformation is protected speech, there could never be any underlying basis for liability under state or federal law for 230 to protect against.
But I don't think you are correct. Knowingly or recklessly false, harm-inducing speech can be a basis for liability notwithstanding the First Amendment (see, e.g., the whole law of defamation), so, even if no such basis for liability currently exists, I can quite easily see a narrowly crafted law on civil liability for harms from misinformation relating to a public health emergency surviving strict scrutiny, with a mental state requirement similar to the Times v. Sullivan actual malice standard (since the subject matter would inherently be a matter of public concern.)