For that experiment to be valid you'd probably want to do a control where you try the stretch, then wait doing nothing for the same amount of time that you would roll your feet and try again and another where you just rotate your ankle in the same way you would if you had a tennis ball under it. Taking your baseline reading one day and trying the tennis ball plus stretch the next day or week would be good too.

This wouldn't really convince me because I find any additional stretching helps with my reach and I'm more likely to chalk it up to a warm up effect or maybe some other effect on the the tendons or muscles. I don't see any particular reason to attribute it to fascia release.