You are applying the logic of a corporation to a non-profit. This is like applying classical mechanics to massless particles. It doesn't work. The price/demand curve is based on competition. Nonprofits are not competing. They are giving it away for free, regardless of the value. There is no price/demand curve for them.
TFA is talking about the "value" of OpenOffice to the world, the value provided by a nonprofit organization.
If a group of doctors volunteer their time and work in a clinic and treat the poor, pro bono, are they not entitled to claim the value that they provide is based on their normal rate? Same question for lawyers who provide pro bono counsel to those who cannot afford it. Can't they claim the value they produce per their normal hourly rates?
No one would argue that the value of their volunteer efforts is zero because their "customers" would not pay the prevailing rate. That is irrelevant, since no one is asking them to pay that rate. It is a charitable act.
The article merely applies the same logic to professionals in the engineering field, whose public service is in the form of publishing open source software.