The company produces some amount of economic output in a year. This can be stuff or it can be innovation. That is the total productivity of the company. The sum of all productivity of its employees adds up to that value. Right now, people are paid less than the value of their productivity so there is money leftover for the company to either stick in the bank (profit) or reinvest in other things.
Not everybody is equally productive. Somebody who has more skill or knowledge in a profession like software engineering will be considerably more productive than a less skilled engineer. Among leftist circles, I think you'd be very hard pressed to find people who describe productivity as literally just "number of hours worked".
What about companies that produce negative value (say, an oil company that caused an environmental disaster)? Are the workers going to return part of their salaries back?
How many software companies got off the ground, hired people, PAID SALARIES, yet never got to see themselves become profitable?
How many companies would not even have existed if there was no risk investment done?
Not everybody is equally productive. Somebody who has more skill or knowledge in a profession like software engineering will be considerably more productive than a less skilled engineer. Among leftist circles, I think you'd be very hard pressed to find people who describe productivity as literally just "number of hours worked".