Poker and Pool are games that both involve skill and luck (just like golf and a hundred other sports/games). The element of a stake is totally irrelevant to that, with the grand exception that almost no one will take the hours and years of practice to be good at either unless their is some potential for financial benefit.
If you don't think there's a LOT of luck in 9-ball, then you don't understand pool that much. I can beat a better player in a race to 7 in 9-ball, and even get hot and beat him a few races in row. But if we play every day, he''ll always win more than me at the end of the year. Poker is no different. I can play an inferior player in HU sit and go's and lose 5+ of 10 games 1/3 of the time, will lose 50+ out of 100 occasionally, but if I play an inferior player 1,000 + times, I'll win every time.