I understand from this answer Three Phase off grid and single phase generator - Victron Community that Switch as Group cannot be disabled in the presence of a PV inverter because the inverter needs to be able to change the frequency for shifting.
So, if a 3-phase system with Switch as Group disabled has an AC Input from a generator on L1, then the frequency of all 3 units will be adjusted to match that of L1 to ensure the phases remain synchronised.
However, I do not understand why the presence of the ESS Assistant means that this function cannot be used. Again, I agree that if the ESS assistant has been configured with a PV inverter, then Switch as Group must be forced on. But if ESS is being used with DC-coupled PV only, why would it not be possible to charge a 3-phase system from a single phase generator on L1?