You thought wrong. Try reading a Louie Simmons or Dave Tate article on the box squat.
And here's one from this site.
"The box squat also breaks the eccentric/concentric chain. This is one of the best ways to build explosive strength. The box squat also causes you to squat from a static contraction to a dynamic concentric contraction, another very effective way to build explosive strength."
Squatting onto a box vs squatting and deloading the weight onto pins in the rack are two completely different movements. The box squat is much more natural and has better carryover to the free squat, the same way that block pulls are more natural than rack pulls and board presses are more natural than pin presses.
The box does a lot more for the lifter than just develop technique. It builds up the posterior chain and many important muscles used in the squat that aren't as effectively built up by free squatting alone.
Using the olympic squat onto a low box would be the best way for it to transfer over to the olympic squat. A powerlifting style box squat would have much less carryover than just performing an olympic squat onto a box would. However, it would have some carryover to the snatch/C&J because of the posterior chain work you get as a result. So, while a powerlifting style box squat probably wouldn't be the best way to drive up your olympic squat, it still would benefit the olympic lifts.
Also, of all the compound movememnts a weightlifter might perform, I guarantee that a powerlifting style box squat would take the least amount of time to become proficient in. That's why it's so commonly used to teach beginners to squat. Someone who's never squatted in their life and is unable to free squat could do a few sets of box squats, pick up the technique and become good enough with the movement to remove the box all in a matter of one workout. It's extremely easy to learn and perform correctly, and anyone who is performing lifts like the snatch or C&J will have it down in no time.