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
The original Model T plant (the Piquette Avenue plant in Detroit proper) was not vertically integrated, and in fact most of the "running gear" (all the complicated stuff, basically everything but the coachwork) was purchased as parts from the Dodge brothers (who owned a plant at what is now Detroit/Hammtramck Assembly -- formerly Dodge Main and then GM Poletown -- a GM plant making EVs like the Sierra and Hummer EVs) and merely assembled by Ford employees (albeit in an admittedly revolutionary assembly line process that changed capitalism forever).
The Highland Park plant was Ford's play to cut the Dodge Brothers out of the process by machining most of his own parts. The peak of vertical integration would be the River Rouge plant which, as you say, machined all its own parts from iron and steel made on-site from raw ore (but never made the Model T).
