He didn't do a bad desk though, so no reason to be salty. It doesn't seem like lot today, but was pretty decent money in Poland 20 years ago. And the same time the devs who bought the rights had only developed a few of point and click adventures (well received in Poland, sure, although nothing really special). The success was far from assured. In fact they abandoned the idea and sold the rights to CD Project, new and untested studio.
Grabbing the cash was pretty smart choice at that point. Sure, now Witcher games are massive, the last game was amazing and basically a new benchmark for triple-A rpgs, but who could have known how it will go. You might as well regret not buying Apple shares when they were near bankrupt or something.
What I think they guy is really angry about is that the games are more popular than the books worldwide. He's got a huge ego, so I doubt that sits right with him. Also, he's not the nicest person - he's comments about his readers can be just as dismissive and insulting as about video games.