But plastics are not made from atmospheric CO2, they are made from fossil hydrocarbons, i.e. from oil.

Not all plastics are made from fossil fuels, and photochemsyn was suggesting: “making plastics from biomass carbon sources or from direct air capture”

That's true, but it misses the point rather badly. A big reason that CO2 is a problem is that it is very stable. Getting it out of the atmosphere at scale requires vast amounts of energy. So if your goal is to reduce the amount of atmospheric CO2 (and if it's not then you've essentially given up on the future of civilization) it is much better not to let the carbon get into the atmosphere in the first place. Carbon in plastic is already sequestered. Dumping it back into the atmosphere is not progress.

That’s perfectly reasonable in the short term. Long term however we don’t want to endlessly extract CO2 either, so eventually we will want a pure equilibrium state with zero net gain or loss in atmospheric CO2.

Yes, of course. But I don't see any circumstances under which returning the carbon in plastics to the atmosphere contributes to that goal.

The overwhelming majority is. So much that it's not even worth discussing the rest since it makes it look like it's not all that bad. Yes, it is that bad.