Every manufacturer is different and the link was only meant to explain why some GFI breaker protection can fail others can fail for similar reasons, but each is manufacturer specific.
We did 50-75kW arrays on three identical buildings in a business park and one of them had a 1600A GFI main that the manufacturer wouldn't guarantee in a backfeed. They all had the same model breaker, but one of them had a different GFI trip unit.
The AHJ in that area was anal about tapping existing gear and required third party recertification so we had to weigh the cost of that VS replacing the breaker. We wound up tapping ahead of the main and having it recertified as that was quite a bit cheaper than replacing then main in that case.
We were lucky on that building as the other two had such crowded electric rooms that we couldn't have tapped (within code distance) ahead on the mains.
We have had another case where we couldn't make the tap length 25' or less and the AHJ was OK with it.
So far we haven't encountered a large fused main with GFI protection and that may be entail an entirely different GFI scheme.
