If you interpret the quote to use the colloquial definition of "possible," but assume that it only applies to things where no elements of the activity are outside your control, then it's deceptively lacking in meaning, because beyond a trivial scale there is very little one can accomplish where this applies.

Fallacy of the grey. An athletic competition involves a great deal of elements where one's control is neither complete nor absent.

it's deceptively lacking in meaning,

If the author had intended deception, he'd have seen no need to include the disclaimer regarding possibility. He effectively said, "if you believe you can, then you can do things that otherwise would be very difficult -- you won't do the truly impossible, of course, just the seemingly impossible."

Since the beginning, not one impossible thing has ever happened. If it happened, it was possible, after all.