Why was this considered "impossible?"

It's a brilliant idea, of course. But being considered "impossible" means it was considered previously and decided to be impossible. No?

I mean, crpytographically, it's ultra-trivial. You "just" need to solve the logistical issues of (1) shortwave radio existing (2) figuring out how to make sure your field agents possess and are not caught with the disposable one-time codes. I am surprised anybody would consider that impossible.

(I hope I am not downplaying the brilliance of the one-time pad idea itself)