Yes, I do believe that poker is a sport.
It's a sport cause you have to prepare yourself mentally and physically.
Mentally by doing some meditation / yoga, taking naps to refull your energy and resting your brain, always performing some maths even if you're not playing to keep all the stuff in your memory "
odds,
equity..."
Physically by doing some exercises, running, going to the gym a couple of times a week cause playing 8-10h straight isn't that obvious for your body and by avoiding sedentarity.
It's a sport also cause you'll find all the ingrediants: Preparation, Goals, Challenges, Competitions...