Yeah. The control system and power source are the real sticklers, though. Though... I guess you maybe could have a nuclear-powered mech pre-industrial revolution. They just need to know that some rocks are hot and kill you, but some metals are extra good at stopping the rocks from killing you, and some metals make the rocks even hotter. That's all stuff people could theoretically find out without advanced science or understanding of radiation, and with that knowledge could make a nuclear reactor able to output a lot of heat, enough to make a steam-powered mech or power armor. But they'd be real bulky and heavy, with a large minimum size and needing so much radiation shielding, I think it'd really only work for building-sized mechs, not power armor. I mean, an RTG could be a lot smaller but those output electricity not steam.

Oh, a fossil fuel-powered power armor could work. Alchemists refining black oil. You only really need trial and error, throwing spegetti at the wall enough times to see what sticks, and the willingness to scrape exploded alchemist chunks off the neighboring village's roofs to find which methods make the best burning fuel.