From the article:
"Do you believe that “what goes around comes around”?
Do you believe that “good things come to good people”?
Do you believe that “people get what’s coming to them”?
Then you believe in karma."
Yeah so, that's not 'kamma' in Theravadan Buddhism. Via , verses 651, 652, and 653, of the Suttanipāta are as follows:
Kassako kammanā hoti, sippiko hoti kammanā,
vānijo kammanā hoti, pessiko hoti kammanā.
Coro pi kammanā hoti, yodhājīvo pi kammanā,
yājako kammanā hoti, rājā pi hoti kammanā.
Evam etam yathābhūtam kammam passanti panditā
By action is one a farmer, by action a craftsman,
By action is one a merchant, by action a servant,
By action is one a thief, by action a soldier,
By action is one a priest, by action a king.
In this way the wise see action as it really is,
Seeing dependent arising, understanding result of action.
In Buddhism, kamma is simply action. The 'proof' doesn't proceed from this definition, so it's inapplicable.