Incorrect. I opened the graphic in Photoshop and checked with the line tool... the hypotenuse is a straight line. It is an optical illusion that makes it appear to curve.
You must not believe your eyes on this one, look at the rise and run of each triangle. If they have the same slope then when they are joined hypotenuse to hypotenuse they will make a straight line. If the slopes are different, (which they are .4 vs .375) then it is photoshop that is misleading. (Perhaps the drawing is not "true" have photoshop replot the shapes from the vertices and see if the original drawing has been "fudged" to make the puzzle harder to solve.
I don't care what photoshop may show... but a 2x5 right triangle does not have the same opposing angles as a 3x8 right triangle. To maintain the same angles, the larger triangle has to be 7.5 units on the adjacent side if the opposite side is increased from 2 to 3 units.
Given a right triangle, the smaller (2x5) has angles of 90º, 21.8º, and 68.2º. The larger triangle (3x8) has angles of 90º, 20.6º and 69.4º.
The differences make up an area of 1 square unit, hence the appearance of a magically appearing "hole."