Dr. A is right that the Supreme Court opinion has a very narrow holding. In fact, it's not even accurate to say that the Supreme Court "reversed" the Ninth Circuit. In the syllabus, you'll notice it says "vacated and remanded" (rather than "reversed") and here's why:
At trial, the plaintiffs argued that the limitations period started anew each time there was a change of circumstances that would force the employer to re-evaluate the merits of the funds. The trial court and the Ninth Circuit both dismissed the relevant claims on the basis that no such change of circumstances had actually been established. According to the Supreme Court, the Ninth Circuit fell into error by deciding the case on that basis without considering the "common law of trusts" which imposes a continuing duty of prudence on the employer.
By the time the case reached the Supreme Court, all parties agreed that the Ninth Circuit's analysis was incomplete. The Supreme Court thus remanded the case to the Ninth Circuit to consider (i) whether plaintiffs had properly preserved the more general argument not based on a change of circumstances, and (ii) if so, whether the more general argument resolved the case in favour of plaintiffs. The Supreme Court declined to opine on either point, noting that, "[w]e express no view on the scope of respondents' fiduciary duty in this case".
The Supreme Court's choice to leave the preservation issue to the Ninth Circuit on remand is interesting because if the argument turns out not to have been preserved, then the Supreme Court actually had nothing to rule on in the first place.
In any case, it's accurate to say that this opinion stands for a very narrow proposition of law, namely that a Court considering a claim for breach of fiduciary duty under ERISA must consider the "common law of trusts" and any duties imposed thereby in disposing of the claim. The core holding of the case was the Ninth Circuit had failed to consider all the relevant law. The Supreme Court noted in several places that on remand, the Ninth Circuit might still dismiss the disputed claims.