The stats I've seen are somewhere around 85% of right handed people will tie a granny-knot one way and ~85% of left handed people will tie it the other way (assuming you know where they were standing, i.e. what their starting orientation was, when they tied it). People who tie correct square knots are more likely to know about knots in general and are thus less likely to care about starting orientation so the correlation is less strong but still more than 50/50.

WRT the "knot only a butcher would know," I'm going to assume it was the surgeons knot because it is in a picture in the article and could actually be used correctly by both a butcher and binding a person. Only butchers and surgeons use that knot because it is used in cases when the rope/thread is extremely slippery as it makes it easier to keep the knot tight in those cases. No one else uses it though because it can come undone easier than a basic square knot if unbalanced as it is in the picture. So I believe that it could be an indicator that the perpetrator was the butcher.

Overall, I'd say less good than DNA evidence, better than phrenology.