Your question is slightly confusing.
The docs there are saying that you can push video frames into either a GL_TEXTURE2D or a GL_RENDERBUFFER.
In the first case, you can use the sampler2D in your fragment shader to look up colors from the video and put video on polyons.
In the second case, yes, you can bind the renderbuffer to your FBO there and get images using glReadPixels().
However, I wouldn't call either of these a "source" necessarily? They are targets. Unless you mean as a source for later reading?