The OP was about the accuracy of the video. I agree with @Klaveness in that it’s a bit confusing because it’s very uneven, good in places, not so much in others.
It starts out with a depiction of the scene in the movie “Battleship” with an anchor stopping the ship, he does say the movie take artistic license which is true but it does hurt the credibility of the video. Not least because in the movie the anchor was used to turn the ship, not stop it, which is better but still not credible. Pay out all the chain at full speed something’s likely going to let go, not sure what exactly.
Perhaps the greatest movie scene ever where the anchor is used as a ship handling aid, with all the usual caveats of course. 
The second issue is saying that the anchor doesn’t hold the ship in place but only holds the end of the chain. Again this is confusing but it’s just playing with words.
If the anchor was replaced with the required 20 shots of chain saying that the last 20 shots only hold the first 12 shots but not the ship doesn’t make sense.
If the narrator had instead said something along the lines of “it’s helpful to think of the chain and anchor as having different functions”, in that the catenary of the chain mostly absorbs energy and while the anchor provides holding power that would have been more clear.