I think the east side Cadillac plant you are referencing was originally a Hudson plant. Cadillac had an east side location on Riopelle, which was also the first home of Detroit Transmission, the beginnings of Hydramatic. Most of Cadillac’s Detroit operations were on the lower west side of Detroit. The Poletown plant, now home of GM’s high-end EVs, replaced it. They also built an engine plant, I think in the late 1970s, in suburban Livonia. That plant built the Northstar engines.
The other interesting historical error with GM is when articles confuse the location of the GM assembly plant located at Willow Run with the bomber plant that became the home of Kaiser and later Hydra-Matic. That plant, located north of the airport, built Corvairs, the Chevy IIs and its various derivations, then full size Chevys before their assembly when to Arlington Texas.
One of the sad facts about Detroit and the environs, is that it is still full of historical industrial buildings and sites with little attention to their historical significance in making Detroit the Motor City.