I believe that the guns in the video were MKII versions. The genius of the STEN was it's simplicity. No great amount of tooling or machined forgings required. Late war time production guns cost $9.00 according to SAOTW.
I think the reports of STENS going full auto on their own or when chucked thru' windows is so much "everyone knows" BS that has been repeated so often, people believe it. Of course the gun fires when the bolt slams forward! It was designed to do so. That's why it has the bolt handle locking notch which requires effort to engage. Take it off that position and you had better keep your finger off the trigger and the muzzle pointed in a safe direction. The pressure on the rds from the mag spring is such that it requires a drop from full #### to strip off and chamber/fire a rd.
The STEN had a button directly under the safety slot to select FA or SA fire. In SA mode, it was incapable of firing more than one rd per trigger squeeze.
I've owned an MP 40 in the past, and while I regard it as a superior weapon (plus the cool factor) to the STEN, I wouldn't feel badly armed with one. It too had a safety notch on the receiver body and no capability of SA fire, although with practice it was possible to rap off single rds. Great fun bouncing tin cans when in this mode!
"WARNING! Always remember that this weapon fires when the bolt goes forward! Never let the bolt go home while the loaded magazine is in position. Unless you wish to fire the weapon, always remove the magazine before easing the bolt home." SAOTW
If you think the STEN was a POS, then you haven't had much experience with the Sterling C1 SMG. It was my issue weapon as a Signal Corps Lineman. How it could have been regarded as an improvement on the STEN escapes me save the fact that it had a combined thumb operated safety/select fire lever.
On annual qualification if I didn't score a 'possible' or as near as damn, I was having a bad day. The course of fire started from a sand bag rest at 100 yds and ended up at 7 yds where you had to fire off all remaining rds FA.