They didn't start a stall manouver, they wanted to demonstrate speed/stall protection.
Which part of "test flight" do you not understand.
Each item on the test schedule must be tested to see if it works CORRECTLY OR NOT. Nothing can be assumed. This is a TEST not a demonstration or a routine ticking of boxes.
A competant test pilot assumes that each and every test could fail and has prepared for all eventualities including the worst case. Starting the stall test at low altitude was a stupid error made by a crew who were not competant test pilots and should never have attemped to fly a test flight.
Scrounging an unofficial copy of the manufacturer's test schedule does not make you competant to conduct a test flight.