In his example, there is no mention of the mdu (bus) rating or its mcb rating. As such, we cannot assess the sum or percentage for the mdu.
Regarding the subpanel, the 20A PVI-backfed breaker would have to be positioned opposite the subpanel bus' service-supplied end, i.e. 100A Panel (Bus) Rating < 100A Service-supplied Breaker + 20A PVI-backfed Breaker.
