It is apples to oranges because the 767-300 and 737MAX are two different beasts.
A reference point from a few years ago shows the hourly block and CASM numbers for widebodies across the major US airlines. That puts the 767-300 CASM at around 7.5 - 9.3 cents.
If Norwegian can fill their 189 seats on the 737MAX, they will make money. Only a handful of those seats will go for $99. As the plane fills up, the prices go up.
Filling up 200-250 seats on a 767 becomes more crucial due to the higher up front operating costs.