It might be valid, but resorting to unsporting tactics such as diving for the line when you're not playing rugby rather cedes the moral victory to the person you just denied the actual victory.
How is it unsporting though? If those who made the rules felt diving for the finish line were an unsportsmanlike move, they would have made it against the rules to do so.
She couldn't beat her opponent physically, so she outsmarted her opponent. Since athletic competitions are all about testing natural abilities, I would say using your intelligence to compensate for any physical shortcomings is still well within the spirit of athletic competition as long as no rules are actually being broken.