Major - P3 The title of the linked section is incorrect:
"Require Replication before Chunk Migration (Secondary Throttle)"
Regardless of the setting of _secondaryThrottle a chunkMove will wait for replication (if it is too far behind) before initiating migration. This behavior is hard-coded.
_secondaryThrottle only takes effect during chunk migration and refers to the commitment stage of the move (i.e whether or not the source documents are deleted from the originating shard before or after a secondary has confirmed replication).
Interestingly, the body text on that page is stated correctly, albeit somewhat subtly.
I think it is worth calling out this difference in wording. Explicitly stating that _secondaryThrottle only applies during a chunk migration and that the chunk move will not be started (regardless) until secondaries are within 10 hours (also hard-coded) of being up-to-date.