This is certainly true heads-up, but Morton's theorem, in which an opponent's correct decision can benefit a player, may apply in multi-way pots.
Suppose Bob is playing limit Texas hold 'em and is dealt 9♣ 9♠ under the gun before the flop.
Even though Carol would still be getting the correct pot odds to call, the best decision for Bob is to raise.
This example also illustrates that one of the most important goals in poker is to induce the opponents to make mistakes.
The fundamental theorem of poker is simply expressed and appears axiomatic, yet its proper application to the countless varieties of circumstances that a poker player may face requires a great deal of knowledge, skill, and experience.