100% Bullshit.
The sanctioning body determines the weightclass for each of it's belts. If the WBC say the weight limit to contest the belt is 160 lbs, then by Christ it's 160 lbs.
Sometimes to make a fight between two guys from different weight classes doable, it's perfectly fine to pick something in the middle. Negotiate what you want if it isn't a title fight, but when you are contesting for a championship belt the weight is governed by the sanctioning body.
If the WBC wants to create a new weight class for Canelo, and make him the champ that's one thing, but if you are claiming to be the MW champion, it's 160.