I’m puzzled as to what is supposed to be the benefit of this excursion. The quick reader might get the impression that the author is trying to say that all sin involving “grave matter” is a mortal sin, but he did not and it is not.
He addresses it in describing the outcome of of VCII where they attempted to redefine sin.
a threefold distinction of sins, classifying them as venial, grave and mortal.
He then quotes JPII who 'clarified' the fact that grave sin = mortal sin.