This is probably because my explanation is very brief. I don't see how a shader (a program running on the GPU) can detect that the OS has killed a process and initiate a clear.
shader is a program executed by the gpu and can manipulate the memory, driver can create a fake surface out the freed memory and run the shader on it (which would avoid the need of zeroing the memory from the cpu trough the pcie)
when you do a release on a texture object, when the context is destroyed, when the glDeleteTextures is called.. you just have to enumerate it all, but eventually all functions are passed to the graphic drivers to be translated into gpu operations.
It's as same as saying that a HDD driver can zero deleted files and delete temporary files when a process is killed because it translates API calls into HDD controller commands.