It never opposed modernism at every turn. That is why so many came and went. It always had this ultimate aim of securing a comfortable niche within the pantheon on terms decided by a very small band of clerics with its own take on tradition. Lefebvre's brand was always there if modified somewhat after his death.
The so-called SSPX division if it exists must be remarkable for its invisibility. The Society has been successful in expelling (or castrating) those Bp. Fellay once said he did not want in "his church". If Menzingen has hardened towards Rome, it will be be part of this familiar game it likes to play and not because of any pressure from the rank and file.
Your first sentence is a factual predicate to the rest. And again, it is right to observe the pattern of appearing to hold firm to principal with the inevitable recession to "openess". It is, as it has been from the beginning. Bp.Fellay is an ambitious man, and he brooks no challenges to his reign or power. Bp. Williamson was the most obvious example of that, but any other voices which have been raised against his vision have been silenced or removed as well.
These outsiders such as this Cardinal and others do not see the long standing familiar pattern of advance and retreat, which has alway been present in the SSPX. They think that these are actually changes in attitudes and position.
Its opposition, or resistance as it is now called, has always been mainly in defiant or dissenting words, while they quietly go about doining what they want to do under the cover of the crisis/necessity blanket.
That the conciliar authorities have tolerated and even treated with them for so long is proof that they were never a serious threat to the Revolution, and in fact the SSPX's very moderated dissent has served to give credibility and legitimacy to the conciliar bandits for decades.
It follows that if there is such strong opposition among some of its priests they would be principled enough to leave straight away, but that is not what happens as they were formed with the stencil of non-threatening dissent which is a hallmark of the parent sect.