It's the United States, not the Divided States, so why are people calling Biden's plan partition?

Answer: for their own political gain at the cowardly expense of the lives of our troops.

There are: The United States of America; The United States of Brazil; and the United States of Mexico. The U.S.A. is not the only nation having the right to call itself the "United States."

Biden's plan simply asks why not let the Iraqis decide to form their own states, limiting the power of a federal government and, in doing so, snatching away the elusive prize of ruling the entire nation from the center.

I think because the Sunni, Shia and Kurds most likely will not be able to work together to be a United groups of Iraqi states. The divide amongst then runs deep and won't be fixed quickly or easily and this plan may help temporarily but probably won't last for the long run. You will also still have the disagreement over how the central government will disperse the oil revenue among the states as well as the makeup/representation for each group within the central government. It seems like the most logical solution but the above is only my guess as to why people would consider it a partition.

If they can't work together, it seems better to keep most of the functions at the state level. The alternative seems to be a strongman - because as you say they can't work together. As the rift heals, they could of course change the plan.

That's cool to keep the functions at the state level but that would also require reorganization of the population into their respective states. Without that reorganization you won't have stable environments to lead toward healing. Besides, a forcefully imposed segregation of the people wouldn't go over very well. You would also still have disputes over representation in the federal government and probably over the oil revenue as well. The oil part may even cause federal government conflict with the state that gets the high oil producing area of Iraq. The state may want to control it and not share.

The USA is a Federal system, but Biden's plan doesn't call for a Federal system of partition. His plan is more akin to a confederate one where the states keep most of the power.

I'm actually surprised it's not the Republican plan, honestly.

Plus, I'm not convinced it would work. The worst violence is in the areas and cities where multiple sects/groups are in close proximity and neighborhoods are a jumbled. The maps of Baghdad play this out. Where once Sunni lived next door to Shia, now there are all these mini enclaves. Who determines the borders? What if you're on the wrong side of the border? How will this plan save your lives and ensure you don't have to uproot and flee for your lives as over a quarter of the Iraqi population has already done?

People call it partition because that's the emotional guts behind it. Separate the groups into their respective areas and keep them as apart as possible, while still unified as little as possible. It's the 'meat' of the plan.