The passive attendance at non-Catholic religious functions was something that was tolerated for a serious reason, rather than simply allowed as a matter of course.
The distinction here is that the Novus Ordo is not merely a non-Catholic religious function, such as a Baptist funeral or a Lutheran wedding, but in fact claims to be and masquerades as Catholic worship. If it is legitimate Catholic worship, then that's where one should be attending Mass, not an independent, canonically illegal venue. If it is not legitimate Catholic worship, then (unlike the Lutheran service) it would be a sacrilege.
That being said, I don't want to derail this thread into another debate about the NO vs Traditional Mass.