Unless pirate players have a significant negative effect on the gameplay experience, which doesn't seem to be the case from what I can see for this game in particular, World Of Goo's makers do not care how many people pirate their game, either in absolute terms or as a percentage. All that really matters is the absolute number of legitimate sales.

It doesn't matter how many people pirate content vs buy it. What matters is whether DRM would have increased or decreased the total revenue. More than likely it would have caused it to decrease. The people who pirated it would not have necessarily been paying customers, and fewer people playing the game overall would likely have led to less publicity and fewer total sales.

Edit: a lot of game makers are adopting this sort of model as an intentional business practice by going "free to play". Having a big installed base is great if per user costs are low (which they can be if everything is digital) and can improve average perceived value for multiplayer games, all of which can make it easier to pull in more revenue from a small subset of players who voluntarily choose to pay for certain things than to attempt to charge everyone a flat fee.