It's not really possible to not know that you are dealing with a runaway stabilizer. MCAS (and every other automatic system to adjust trim) causes large physical wheels at the side of the pilot's knee to spin.
And these large wheels have small bell/clackers on them so you get a distinctly audible signal in addition to the large black wheels with white stripes on them.
Disregard the text on the video description as it's blatantly wrong (describing that the trim tabs still move without the wheels moving; wrong on two accounts: the trim doesn't move without the wheels moving and the 737 uses a jackscrew for horizontal stabilizer trim rather than trim tabs [which is why the pilot can't simply override the aerodynamic force as they could with a trim tab])
The 7-3 has balance tabs to make the control forces lighter in the event of a hydraulics failure, but these are not trim tabs in any sense of the word.