A spectrogram is literally the same audio, just transformed through a Fourier transform. That transform has a trivial inverse. The spectrogram isn't perfect - t

A spectrogram is literally the same audio, just transformed through a Fourier transform. That transform has a trivial inverse. The spectrogram isn't perfect - the visual representation is low resolution and the phase information is missing - but it's plenty enough to at least figure out what was said. There's nothing surprising that this is possible, only disappointing that whoever published the article didn't realize it.