Oh, no, that's not true at all. An invention might not even be possible, let alone having a clear and (literally, provably) correct specification of inputs and outputs. Just seeing that it exists, to say nothing of getting the correct proportion of inputs and outputs, could shave a decade off your invention cycle.

Take, for example, the nuclear bomb. Just knowing that it could be done put you ten steps ahead. What if cold fusion or a warp drive were known to be possible because you could see it (even if from a great distance with little detail)? Airplane manufacturers leapt ahead (literally) after the Wright Brothers.

A tremendous amount of effort for worthy inventions is often involved simply in proving that it can be done. Once you know it can be done, you don't have to prove it anymore, and also large companies will throw buckets of money at a clone of something that's proven to work.

A patent (sometimes) prevents that -- at least, when everything is working as it should be. (In this case, clearly not!)