Maybe I was being presumptuous by assuming that the signals pins == "signal circuitry" in the diagram.
@nquery You were. the lines from the bottom of the 'signal circuitry' box are ground connections. The signal passes from left to right in the diagram.
This document relates to how EMI circuitry should be used in balanced connections to deal with high frequency issues that may result in noise. So it shows a variety of hookups and shows how a number of them can be problematic.
In any even while the signal circuit does use ground as a reference, the signal current does not enter or use ground. For example in our preamps, which have a patented direct-coupled balanced tube output, if you ground one side or not, the signal level remains constant; IOW is the same amplitude.
I'd be very interested to see a circuit where signal currents are not present in the ground, yet when operated balanced you get double the output you do single-ended!