I don't know who else might have done it, but I don't see it as a mistake. Note the OT is wired with C (common), 4 8, and 16 ohm taps. The speakers themselves do not reference to ground. You are used to seeing one side of a speaker grounded, but they only do that for circuit convenience.
The voltage between the 4 and 16 ohm taps is what they use as NFB. So they grounded the 4 ohm tap to reference the 16 tap signal to ground for NFB. You could pick any tap you want to ground or ground none at all. They chose 4 ohm.
It also seems that they used the arrangement to create a center speaker signal.
But the first most essential question to ask yourself is this: what problem are you having that rewiring this thing would solve?