There are so many factors that influence your ability to relaunch a kite in lower winds, the main one, as usual, being your skill level.
It's important to understand that the wind can be a lot weaker and more turbulent at the surface of the water. It might be 10 knots up where you kite flies and at the height of your local wind meter, but it might be as little as
6 knots or less at the surface of the water! Here is a good example of this:

That day it was probably around 7-9 knots in the kite, but around 2 knots on the surface of the water. I initially didn't even think the kite would fly, had to launch it straight in the power zone by running backward. No chance anyone can water relaunch any kite in this wind, even if you were able to stand in shallow water.
Then comes multiple factors like: are you standing in shallow water? are you in deep water? is there strong current in the water? waves ? is the wind gusty ? is the wind dense and cold or hot and weak?
So if you actually had 10 knots at the surface of the water with your 11m Rebel and you had solid skills you should be able to relaunch it in deep water provided there isn't a very strong current going in the same direction as the wind. A Newbie in his first lesson could relaunch an 11m Rebel with 10 knots at the surface of the water if standing in knee deep water.
In other words, when you're kiting in 14-15 knots, you're probably actually trying to relaunch the kite with only 10 knots at the surface of the water.
The video you posted above shows the 5th line technique which is essential for C-Shape riders but not as useful/effective with modern Delta kites. By the way, in the actual wind conditions of that video, there was no need to use this technique especially while standing.
Christian