SamplingBasedSplitPolicy::make() sends the $sample random cursor aggregation to the shards as soon as the policy is built, before anyone knows whether split points will be needed. So it runs even when numInitialChunks == 1, or when zones already provide enough split points. If $sample fails in that case (for example with error 28799 "could not find a non-duplicate document" as in BF-46434), the whole reshardCollection fails for no reason.