Ok. So this speaks specifically to the “readback from FBO” case (when MSAA is enabled) and has nothing to do with “readback from the default/system framebuffer” (window, pbuffer, etc.)
That’s a separate question unrelated to the above. I don’t see a clear answer in the spec. The behavior of the default framebuffer is left to the window system. The spec does indicate that the default framebuffer can be read back (at least under some circumstances). But I don’t see a statement on behavior when reading back from the default framebuffer if its MSAA. If there isn’t one, this behavior is window-system specific.
In practice, I think I’ve seen some drivers do an implicit downsample before the readback in this case. But AFAICT, that’s not required by the GL spec. Possibly driver-specific and/or window-specific behavior.
For the “in other words” part you put in the parenthesis, I think you meant: “the effective value of SAMPLE_BUFFERS for the read framebuffer is one.”