Complex sentence: Mr. Szameit also convincingly demonstrates that the sight-size method does not have deep historical roots and that the claims made of past masters having used the sight-size method are incorrect and based on a misunderstanding of the definition of the sight-size method.
Split into simpler sentences: Mr. Szameit also convincingly demonstrates that the sight-size method does not have deep historical roots. He explains that those claiming past masters used the sight-size method are incorrect, basing their claims on a misunderstanding of the definition of the sight-size method.