I looked at this and thought, "This is purely about making people exercise more because it's healthy, not because it's particularly good at relieving stress.
On Monday, I had an especially stressful day, and arrived home an hour and a half later than usual in a foul mood. Lut was the same way, over much the same problem (car troubles, mostly now resolved, happily). I was tempted to skip my workout. It was late and I was grumpy and it didn't seem worth the time.
But I thought, "If I skip working out, I'm just going to feel guilty for not exercising. All I'll do is play games, and that's not that relaxing."
So I went directly into my usual exercise routine.
By the time I'd finished stretching, I'd stopped thinking about my problems. When I was done with the workout an hour later, I was in a good mood again. Perspective had returned, everything was going to be fine, and I was no longer upset or irritable.
So I guess there's something to it after all.
The irony here is that "I'm too stressed to exercise" was the first reason that popped into my head for skipping it. Too pressed for time, too much to get done, making time to exercise is too stressful. Only a couple of hours to try to relax in, can't waste it exercising. And yet exercising was more relaxing for me than anything else I could've done in an effort to relax.
Gotta remember that.
It still seems strange to me that exercise relieves stress, though.