I don't think the part you highlighted is relevant to impartiality. It sets an aspirational goal of Section 230 and the safe harbor is the means to promote that goal. Without the safe harbor, providers would be forced to strictly police what users post. Smaller providers who aren't able to bear the burden of strict moderation would be forced to shut down entirely. This would have the consequence of reducing overall user access to information.
> I don't see how you maximize user control over the information they receive if information providers are going to be partial to one political party.
By using a different information provider.
> description of the internet as, among other things, "a forum for a true diversity of political discourse".
You said it yourself. The Internet is neutral, according to Section 230. Not individual information providers. If you don't like what one information provider does, go make your own. It's all based on free tools and open standards. No one's stopping you.
Believe me, no one of any political stripe should want to take away ability to moderate Internet forums. It will turn all forums, regardless of political, religious or other affiliation, into spam-filled, unusable messes. That also reduces "user control over the information they receive", since the garbage will drown out anything useful and platforms will have no power to remove it.