Yes, Ford claimed up to 90% in the 1930s when they were producing up to 1.4M cars and trucks per year. (Down to less than 400K in the worst of the depression.)

I suppose it makes sense, back then you wouldn't think there would be an existing supply chain of companies like Mopar just waiting for a car manufacturer to spin up and start buying their stuff

They would go so far as to license components for in-source production from suppliers who couldn't meet the needed volume.