As far as I am aware, both are right, they are talking about two different scenarios.
If you switch between fewer rendertextures (of same size) than the maximum number of FBO-attachments, you can attach them to a single FBO to switch quickly between them. This is a static attachment, typically done at init-time, it never changes.
What Valve are talking about, is swapping out the attachments, which does not apply to the situation described above.
With "switch quickly between them" you mean attaching them once, and then just setting the one to be used with glDrawBuffer() ?