The new understanding of the generation.
I know the WT uses the "other sheep" and GC interchangeably to refer to Witnesses in the "Last Days" but strictly speaking the GC are simply those people who are survive through the big A.
Redefining the generation redefines the whole thing. The quote reflects an old understanding of how the "generation" affected the understanding that the other sheep of 1914 would still be around to survive into the new order as the "great crowd".
Pointing that out to any Witness will simply result in them walking around the boulder of cognitive dissonance.