The capacitor provides a signal path from the second grid to ground, which is needed for balance of the PI. Without it the grid wouldn't move relative to the cathode and there would be no output from the second PI section.
It doesn't matter whether you connect it straight to ground, or couple a signal in through it, as long as the reference is there.
When global NFB is used, the capacitor is one of the time constants in the loop. If you make it the same size as the PI input coupling capacitor, you can get motorboating. So, designs with NFB oversize it to 0.1uF. The AC30 didn't have NFB.