He says that one "should" not attend because it might endanger the faith, but there's no objective standard there for making attending the NOM unacceptable. I could attend the NOM with zero danger to my faith.
He made the objective argument 12 times in 11 minutes in Mahopac. Perhaps you (needed to?) miss that?
From the Mahopac conference video:
1:02:17 - "There's the principle and there's the practice. In practice the new Mass is a key part of the new religion, which is a major part of the worldwide apostasy of today." Conclusion: The new Mass is bad.
1:02:34 - "Archbishop Lefebvre, in public, would say stay away. Keep away from the new Mass." Conclusion: The new Mass is bad.
1:03:10 - "In certain circuмstances, like those you mentioned, exceptionally, if you're not going to scandalize anybody..." Conclusion: The new Mass is dangerous.
1:03:29 - "The conclusion many of them are going to come to [i.e., people who see you go to the new Mass] is that the new Mass is OK." Conclusion: The new Mass is not OK to go to.
1:04:35 - "The principles are clear, and the wrongness of the Novus Ordo Mass is clear." Conclusion: The new Mass is bad.
1:05:00 - "The Archbishop said if you want to look after your faith, stay away from the new Mass." Conclusion: The new Mass is bad.
1:08:40 - "The new religion is false, and it strangles grace." Conclusion: The new Mass is bad.
1:10:30 - "But I hope its clear that I don't therefore say that the NOM or Novus Ordo religion are good; that's obviously not the case." Conclusion: The new Mass is bad.
1:10:40 - "Generally, it’s a tremendous danger because the new religion is very seductive...and it’s very easy to go with it and lose the faith." Conclusion: The new Mass is bad.
1:12:24 - "Stay away from the Novus Ordo, but exceptionally, if you're watching and praying, even there you can find the grace of God." Conclusion: The NOM is dangerous; stay away from the NOM.
1:13:24 - "But it does harm in itself, there's no doubt about it." Conclusion: The Novus Ordo is bad.
1:13:45 - "It’s a rite designed to undermine the Catholic faith." Conclusion: The Novus Ordo is bad.