Neither one. Think of the SCRs as gating the generator away from the battery. Apparently there is some problem with simply opening the legs, which could be easily done, so instead they're grounded. The rectifier keeps it from grounding the battery.
Back-EMF, etc.
That was my point about the electronics of the day being too fragile to handle switching currents of the type we encounter here, and quite probably why, if they existed at all, only to be found in military applications where the taxpayer was footing the bill. Shunt (or short) regulation was a cheap and
simple way to handle it instead. Even though it seems crude now, it was actually a clever solution - for a while (my comment about warranty period applies). This situation continued for decades, until cheap and reliable high-current semiconductor devices were developed.
Thus, we have the consequent development of the series regulator...
The series regulator cuts it off entirely, saving the stator from working at full tilt all the time. The stator is happy, the regulator is happy, and the rider is happy.
Even at that, the first proper mass-production series reg/rec wasn't taken up by any of the Big4 bike makers, but by Polaris for their ATVs. It was a resounding success and grew from there. Polaris had a real problem with their PMAs and shunt r-rs, with damn near every one being warrantied, so they were happy to try the Shindengen SH-775. It certainly got them out of a jam.