Is it bad advice? Yes. It's so easy to get a knot out of a line. Why would you not do it? I've unknotted lines for a mate on the beach before a session. The knotted line probably won't break, but it might.
Your math is sound, sort of. Kite lines are generally 300kg breaking strain and can be up to 500kg so it's reasonable to assume 150 kg remaining strength with a knot. But a powered up 80kg rider could conceivably do a 2g load and pop. That's 160kg right there.
Select to expand quoteDEECEE said..
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Interesting point from mark50 about manufacturers only using specific knots in their line setups. ...
Pigtail lines are usually 3mm Dyneema. That's at least 700 kg breaking strain. You can get similar stuff with 1100 kg breaking strain. Even cheap stuff with a knot in it is going to be stronger than your kite lines.
If you make your own leaders and stuff you can double splice them so there's no strength lost at all.
BRILLIANT KIT33R!!! 
I think you have captured the whole concept of the knot / not issue perfectly 
