One might notice, however, that TIA was not primarily answering the question alluded to in the thread title, whether it's a mortal sin TO attend the New Mass, but, rather, they were answering the question whether it's a mortal sin NOT to attend the NOM if it's all that's available to you on a Sunday, i.e. whether the Sunday obligation requires attending the NOM. They give a clear opinion on the matter that NO, it is not because the New Mass is "an activity that has the flavor of heresy and for this reason is highly offensive to Our Lord."
So ... "he must avoid going to this Mass so as not to fall into the sin of promoting an activity that has the flavor of heresy and for this reason is highly offensive to Our Lord. Therefore, he does not commit mortal sin by missing the Novus Ordo Mass."
It says here clearly that if someone KNOWS about the problems with the NOM, then "he must avoid going to this Mass". This is the subjective vs. objective distinction, the stipulation that if the person knows and is aware of the problem with it.
So the Mass itself, they say, is "an activity that is highly offensive to Our Lord" (objectively very bad) and so one must avoid it if one knows it to be bad (subjective).