I guess for him it isn't.
For me, it is the case. If I don't go out and do things, then I have nothing to write about. The more I am out, mixing with people and observing life around me, the more my brain is stimulated. The synapses begin firing and suddenly worlds of ideas shoot across my brain. If anything, I have to be careful of overstimulation.
Although I guess I do agree with him in one respect; playing frisbee won't necessarily give me frisbee-related thoughts. The brain might go down the garden path of hang gliders, or wonder why I do some things better with my left hand than my right. I'm likely to be reminded of years of summer camp, or just hot days in my parents' yard. And the people! People do such interesting things! Perhaps in our time together they will reveal some of their character or their struggles. Maybe we will accidentally find out they have a temper if things go wrong or are unbearably celebratory when things go right. I might see a way to take a character a whole different direction, just because I played frisbee with someone I don't know.
Regardless of which way my brain goes, it won't go at all if I don't have something to feed it. And since I haven't been out a lot this week, the well is beginning to run dry. Good thing the spring semester starts next week.