Doesn't the protection usually work such that it prevents reading the firmware but still allows you to erase and reflash it?

Assuming the other commenter is correct and the mcu is a clone of an ST product, then it's possible that the protection are fuses that destroy the pathways to the memory. They're one-time writable and cannot be undone. At my work that is how we protect our firmware with a similar ST product.

I'm not sure how it works in-silicon. Would be interesting to know how... but it's sunday afternoon