Body building is an aesthetic pursuit not an athletic one. You cannot use one data point like Marit Bjoergen and build a training model on it. While XC skiers do have more upper body muscle mass than say runners, they do not look anything like body builders nor do any successful XC skiers training using body building methods. The point of body building is “form over function”. For every athlete regardless of sport, it is function over form.
XC sking has lots of elevation change in the races. Having to carry excess muscle mass up those hills will be a problem. Instead of looking at the outliers like Bjoergen look at the typical WC skier and you will see something close to the ideal body type for skiing. It would be like looking at Petter Northug’s technique and thinking that was the optimal XC technique! Northug excelled despite his technique not because of it. Copying outliers can be dangerous.