It does. An overhand knot still cuts the line strength significantly. Sleeving, since it isn't actually attached to the line, adds nothing to the strength. It only increases the bend diameter by a few percent. So, smartass, why don't you do the tests and see just how much difference the sleeving makes in breaking strength.
Also, FWIW, breaking strength aside I would NEVER run knotted flying lines. I kite in the surf and often have many twists in my lines, the opportunity to have a line hang up on a knot and send the kite into a death loop is just too high. I had it happen back in the day when we were experimenting with Q-Power line and never again.
Lots of people use line extensions , for an inexpensive way to get a wider
variety of wind ranges ; and i think it's Great !!
but
the damn things always tangled up on me at the knots , if the kite gets dropped
in the water; that is why i just made different length lines, from Q-power , with no
knots in the middle.
I use 23M, 27M and 32M lines on (3) different bars ; and the wind-range for any
specific kite , is definitely increased.