This is a fundamental principle of NMR spectroscopy: chemically equivalent nuclei (isochronous protons) do not exhibit spin-spin coupling with one another, meaning they do not split each other's signals.
The claim states a fundamental, definitional rule of 1H NMR spectroscopy: magnetically equivalent nuclei do not couple with each other to show splitting. None of the retrieved papers specifically address this basic spectroscopic rule; it is an established textbook fact of chemistry (common knowledge).