I wonder how he defined "difficult"? Because by most definitions, I don't believe there's any way pool could be considered a "difficult" sport.
Good question. Regarding these types of discussions, I usually define 'difficulty' as such...
Take a young adult (say, 18-years-old) of average athleticism and hand-eye coordination who doesn't regularly play the sport/activity and see how 'natural' or 'competitive' the person would appear playing the sport/activity after regularly practicing it for a year.
Given that definition, I don't think pool is in the vicinity of being 'difficult' at all, at least compared to other sports such as baseball, ice hockey, tennis, and golf, in which playing the sport at a very early age is critical at playing it competitively. A person who has never thrown a baseball or never skated on ice won't look 'natural' in either of those skills after practicing for only a year, possibly never.
On the other hand, it is very conceivable that someone who has never shot pool before can reach B player status and capable of running a rack after a year of solid practice.
I understand there are other ways to define 'difficulty', but I usually default to the above definition when trying to compare a wide variety of sports/activities.