I am not sure about the exact wording here, which _might_ cause problems if there are GFCIs on branch circuits. However, as far as the _reason_ for part 3 of this article, GFCIs on branch circuits are not relevant.
The issue is ground fault detection on the entire service. A bond between neutral and ground at the subpanel would be a connection between circuit conductors and ground downstream of this ground fault detection. Depending upon how the ground fault detection is implemented, such a bond could desensitize the ground fault detection or could cause false tripping.
I rather doubt that this residence has service ground fault protection