Why are there so many excessive medical claims for hatha yoga?

This is because those who claim much have not studied anatomy and physiology and medicine. For example a question was raised if some mudra can reduce cancer cells. The interplay of cells in immunology is so complex that as physicians we need to read the subject several times. There are cells that activate other cells which in turn activate other cells which in turn activate some receptors and so on. Then there are antibodies to the receptors that prevent the action that is normally intended and then mutations on the receptors and so on. In such a complex situation its not possible to wave a magic wand ( except by God) and manipulate things as we wish. This is reality.