Bear in mind a lot of catabolic pathways hydrate double bonds, and that water can in principle come from the electron transport chain. So that oxygen ends up attached to carbon, and can end up in CO2.

Also pyruvate decarboxylation (by pyruvate dehydrogenase) is not the only place CO2 comes from. The citric acid cycle releases two equivalents of CO2 for each equivalent of acetyl-CoA added. Pyruvate decarboxylation releases one equivalent of CO2 for each equivalent of acetyl-CoA generated, so two thirds of the CO2 comes from the citric acid cycle here (makes sense, since pyruvate is a 3-carbon unit and only one carbon is initially lost as CO2).