Let's see here......
Pythagorean theorem: the square of the length of the hypotenuse equals the sum of the squares of the lengths of the other two sides.
10 feet from floor to rim. X = 10
13 feet from freethrow line to rim. Y = 13
X x X = 100
Y x Y = 169
Square root of 269 = 16.41
So. free throw line to rim, at the shortest possible angle, we have 16.41 feet.
Wilt was 7'1" or 7.083 feet long. But we have to consider wingspan, not height. Cavs tells us Chamberlain's wingspan was 7'8" or 7.67 feet.
So, if the player travelled along the shortest path - directly along the hypotenuse, from free throw line to rim - we get:
16.41 - 7.67 = 8.74 feet
Naturally he has to not just touch the rim but dunk the ball, so we add that in too: 9 inches or .75 feet.
8.74 + .75 = 9.49 foot leap.
Of course, he'd have to get 9 inches higher, to get the ball over the rim.
The standing long jump record is nearly 3 feet further, at 12.17 feet.
But that is feet leaving, and feet landing.
A guy sprawled out trying for reach
would not have to go so far with his feet in order to make the basket. And of course, the claim is with a 3 step leap, not a standing long jump. Spring is vastly improved with steps.
Looks possible from the pure math side.
All that is aside from the fact that the chairman of the NCAA Rules Committee, Tex Winter, went to KU, watched Wilt Chamberlain dunk free throws and got the NCAA rules changed to outlaw... dunking free throws.