No, it's a blatant ad hominem.
The guy invested his time and effort trying to improve the world by writing a technical book, which he then proceeded to give it away for free, and to this we see people like barbs replying with personal attacks accusing the author of being mentally disturbed to the point of requiring therapy.
This is a personal attack at its worst.
Perhaps the issue here is the C programming language and how teaching it can be improved, not what insults and personal attacks a random user online is able to throw at the author of a technical book.
People have more to lean from writing on undefined behavior than puerile complains regarding comments on penis sizes and ironic accusations of immaturity.