Personal anecdote: It seems that some people produce metabolites that disrupt their sleep when they exercise, even at low levels of exercise.
There is the concept of "hypermetabolic" where it is hard to sleep after sustained exercise essentially because your body is so fired up. I have experienced this, but it really has to be an unusually long run, at least 30km usually more. It would not surprise me that you're correct, and the threshold for this state varies by person (and almost certainly fitness level) so for some it could occur at low levels like you say.
I get something like this even from a relatively light exercise (~40 minutes of pilates reformer). If I do it after 6pm my body feels very hot and I have trouble sleeping even 5-6 hours later.