In a sense, SCOTUS justices' opinions are automatically "true" (or at least, the majority opinion).
The law is not a reflection of an external reality. It's an artificial, self-referential construction. Whatever the highest legal power in the land says is true is legally true. SCOTUS could rule on a law in a way plainly contradictory to what is written in law or the Constitution or evident in external reality, and that is law. There would undoubtedly be a cost for SCOTUS to do so - ramifications in terms of loss of trust in SCOTUS and potentially even impeachment of justices should it offend the government - but as far as the law is concerned the matter is now settled. The only mechanism to overturn that is send a law back to SCOTUS to be reviewed.
This is the point of stacking courts and why authoritarian regimes do it - because even authoritarian regimes like to present the image that they are doing things fairly and by the book.