It's not that shocking except in hindsight. The speed trim system is, from what I can tell, incredibly similar to MCAS at least on paper. They both adjust the stabilizer trim in order to ensure the legally-required amount of force is needed to move the stick in certain scenarios, both activate 5 seconds after the last manual trim input when flaps are up and autopilot is off, and both rely on non-redundant sensors. The only difference is the exact combination of sensors used and the exact obscure scenario they're designed to protect against.
I read here previously, I think it was here, that the primary difference was that after repeated counteraction by the crew the speed-trim system would give up, whereas MCAS doubles down and reduces the time between its attempts to force trim "correction", literally wearing pilots down physically until the system wins and the plane crashes.
Another post showed how AoA sensors (IIRC) has systematic errors causing MCAS to operate when corrections weren't required. As you say, lack of redundant sensors.
