with other voting systems, third parties could gain more power (as intended), which means that the current duopoly would lose power, and they don't want that
Probably just a bunch of bullshit falsehoods, look at the campaign against issue 1 in Ohio. It was a change to the state constitution to put a citizen run commission in place to draw maps for voting districts and Republicans ran a campaign that was just straight up lying about what the issue was about. Then even threw misleading ballot language on the ballot, I doubt anyone read the ballot language though because it was a small novel.
Issue 1 took up the entire back page of my ballot yesterday and was just nonsensical legalese. People voted on it based on how there prefered candidates told them to, and we all saw who their preferred candidates were last night.