Most often, a timing belt will not show any signs at all of fatique, cracking, etc. It may look perfect up to the nano-second that it breaks.
While I did not look at the site to see what kind of free tool this is I do not see any tool existing that will determine the condition of a timing belt.
However, I could see a tool being used to check belt deflection based on wear and/or stretch. If this is what the tool does then that’s a pretty shaky method to determine whether the belt needs to be replaced or not and certainly not a method I would rely on.