The numbers and classifications come from publicly available data of Statistics Canada.
.
All businesses claim that they have "multiplier effects" and "synergies" - sometimes true, usually  hogwash, always overstated, as you note.
.
That kind of analysis has to adjust other factors and assumptions.  There are opportunity costs associated with investment in any sector.  Untangling them tends not to be a trivial task in economics.
.
Example:
.
All long term economic gains have come from gains in productivity, which are in turn nearly the result of innovation, not the result of digging things out of the ground - Adam Smith pointed this out, turgidly, in the Wealth of Nations in 1774.
.
So, when we bias our tax system to favour mineral extraction, what is the opportunity cost in lost innovation in services and manufacturing of that implicit subsidy?
.
The cost is certainly large, but its not remotely an easy question.
.
The co-efficient for oil & gas in Canada?  Probably about negative 3.