This is true, but it illustrates a key dynamic. For most people, as soon as you put a dollar value on a task that someone may have done for free or for fun, you have now fundamentally changed that relationship.

If someone is doing something for only the intrinsic motivation, putting a dollar value on it changes it to an economic calculation. The person may still do it for money, but they're more likely to only do a level of effort commensurate to the economic reward. Whereas if they're doing things for intrinsic reasons they may be willing to do more work.

There are a couple of behavioral economics experiments that bear this out. The Soma Experiment from the 70's gave participants a puzzle game to solve, and measured how long they tried to solve it. One group of participants was paid for their time, the other wasn't. The paid group on average spent less time trying to solve the puzzle than the group that wasn't paid. There are a couple of other experiments in that vein, but the common thread is that intrinsic motivation can be more powerful than economic rewards in multiple contexts.

So someone might be willing to contribute some code to OSS for free, but if you ask them to develop some code for $30/hr they might pass.