While technically, per Code, that is correct. But I have always considered this requirement for upstream bussing and feeders beyond the subpanel feeder OCPD an error in thinking.
When a compliant main panel is commissioned, it includes the calculated load of the subpanel. When PV current is backfed to the main, none of subpanel loads are using power from the main. As such, the total current on the main panel bus is reduce by the amount not used by the subpanel and increased by the backfed amount. In a compliant system, under normal operating conditions, the current on the main panel bus will never exceed the bus rating.
PS: I'm doubting the amount is 43A. The value applied to this determination is the OCPD rating, which 43A is unlikely, being a non-standard OCPD rating.