If they permit it then especially in cases where there are no TLM options then it must be that it is not a mortal sin to go to the Novus Ordo mass. Also it should be noted that while Matthew supports the TLM he has also said, But among those who claim to be against Vatican II, there is one group that really boils my blood, and that is the "dogmatic home-aloners". " This also implies that in cases where there is no viable TLM option where you live or in your personal circuмstance then it is ok to go to the Novus Ordo.
I've read that thread. That is manifestly not what Matthew meant. I know what dogmatic home aloners are. They're strict sedes who for some reason don't believe independent/sedevacantist priests have supplied jurisdiction to administer the sacraments, so they basically think we should just stay home and pray and not attend mass anywher,e period. There is no way Matthew had in mind attendance at the Novus Ordo.
And for what its worth, I don't believe its a mortal sin to attend the NO in most, if not all circuмstances. For it to possibly be a mortal sin, I think you'd have to not only be aware of the problems with it, but you'd also have to be solidly convinced in your conscience that the service is an out and out sacrilege, and to the point where it truly is more pleasing to God to just stay home, but choose to attend it anyway for no reason. That's *my* position, and I'll probably get some flack for some here. But whatever your view, its dishonest to quote at least Bishop Williamson or Matthew in this fashion, and I suspect Fr. Pfeifer as well.