As I recall Tesla was talking about enormous amounts of so called cosmic energy, freely available if we just construct a device, which collects it.
This idea seems plausible even to a non rocket scientist, for example, imaging kinda radio waves or electromagnetic field in the air, you build up an antenna to pick it up and here we have free energy from outer space.
I even got to some web sites, offering great Nikola Tesla secrets revealed for some hundred bucks or so. They gave you a free example, a simple device made out of a dozen electronic components, few capacitors, resistors, germanium diodes and antenna, which produces measurable voltage on the output. Many also built this device, and considered it a prove for even bigger device can be built.
But all these offers have just one goal - to convince you to buy the book or downloadable document on how to construct free energy device...which of course DOES NOT WORK. But you spent a hunderd bucks on it and that principle of economy works :)
But was Nikola Tesla wrong?
Well, I haven't studied his work yet, but I can say for sure - NO, he was not wrong! Energy transfer beyond known transfer media is possible. But not wirelessly, as many may believe, nor via fotons or other stuff, known to Einstein Relativity.
I'll explain more in detail, but it's late nite here, me sleepy so came back in few days, and I'll write some more.