I am not entirely sure that the knot slipping is what is perceived as stretch. Using the Gibralter string, I have tried various methods of routing the cord through holes. Instead of simply running through holes and tying, I have also tried running through the holes, crossing the ends of the string across the strainer to wrap around it, then bringing the ends back together in a square knot. Theoretically the additional friction placed on the string in multiple locations prior to the knot reduces the pressure on the knot drastically. Doing this has not solved the issue for me with either construction cord or Gibralter's string. Simply tying a standard square knot with the Ludwig cord works well for me without any additional friction. That is just my experience but that is also why I am switching strainers and utilizing fabric straps on all of my snares.
By the way, I would be concerned that anyone's wire snare cord (whether coated or not) would potentially damage a wood snare bed over time. I have no experience with this, but I bet the cable is harder than the mahogany, maple, birch, etc snare beds.