The original is , I believe correct.
For exammple wages ...assume due to (i) rate of pay , r, Ii) hours worked. t.
So W propto r. And W propto t
Then for this simple case infact consta nt of proportion = 1
W propto r*t
W = r*t

Triple the rate of pay, double the hours worked, then W is now six times larger!
It is. Correct to then state if W propto r And, independently, that W propto t, that also W propto r*t

It would not be correct to state W^2 propto r*t. .
The original author statement is then correct.