I think he means that he stated the result in a calc class, not that he proved it. It would be very reasonable (and cool!) to state without proof in a calc class.
(I am curious what "the resulting corollaries" are, though.)
https://www.math.ualberta.ca/~xinweiyu/117-118.14-15/2014090...
And then the homework (Q3): https://www.math.ualberta.ca/~xinweiyu/117-118.14-15/2014090...
Where the corollary follows.
I still would be hesitant to talk about this in my own calculus courses, but, given the impression that this made on you and Will Stein's endorsement (https://news.ycombinator.com/item?id=12886751), it sounds like that's just undue timidity on my part.