You might want to consider that human behavior can be difficult to calculate on an abacus.

Beginning to realize success toward a goal with a single behavior change (changes due to an increase in exercise) will having reinforcing effects on other behaviors (such as dietary choices).

By gamifying a key factor that likely led to a cascade of behavior changes, the watch played a critical role in improving his chance of success toward a goal.

Whether accurate or not, your point does not negate nor diminish the value of the watch as the critical factor.