A company with long-outstanding bills owed by its customers can assign those bills to a collection agency that pays the company a fraction of their amount and then tries to collect payment from the customers.
Conclusion
Companies with a lot of stale debt should not sell that debt to a collection agency; they should try to pursue collection of that outstanding debt on their own.
Evidence
If you sell the debt to the collection agency, you only walk away with 15% of what the debt was worth
Evaluate
Let's understand the math involved: if your company is owed $100k in long overdue unpaid bills, you can try to pursue that debt on your own (investing money into tracking your debtors down and harassing them to pay you back).
Or you can just offload that chore onto the collection agency. They'll pay $15k for the rights to those bills. (and then they're hoping that they'll be effective enough at harassing debtors that they collect a bigger share than 15% of the unpaid bills)
If you were trying to make a counterargument and suggest a company should go with the collection agency, you'd probably say:
look, I know it's depressing to only get back $15k when people collectively owe you $100k, but $15k is better than nothing. Do you think you'd beat $15k if you pursued this debt on your own? And how much effort and resources would that take?'
Goal
Since the argument is comparing two options, pursuing this debt on your own vs. selling it to collection agency for 15% its value
an author concluding Option 1 is the way to go must think that Option 1 achieves a better outcome. The outcome in Option 2 is 15% the value of the outstanding debt.
So the author is assuming that a business would end up getting more than 15% of the value of the outstanding debt back if they pursued that debt on their own.