Some people do not seem to understand the term "quantum efficiency" (QE).
The quantum efficiciency measures the fraction of photons that are actually detected by the camera.
An external quantum efficiency of 50% means that 50% of all incident photons are converted into electron-hole pairs and can be detected.
There are, however, loss mechanisms that prevent all e-h pairs to be collected. But this is not off by a factor of 1000x from the theoretical limit.
As already stated by the original poster. This figure is probebably for some other wavelengths, like far infrared, where silicon is "blind" due to its band gap.
Since humans are very blind to this wavelengths as well, the relevance in the cameras is questionable.