The most likely explanation I can think of is steaming toward the center of the channel between Henderson Island and High Island (along the heading of what is now the Alaska Marine Highway/Sand Point Ferry ship lane), but that strikes me as a little odd to describe it thus, as the channel would be more accurately described as splitting Popof Island and Korovin Island.
The other plausible explanation (to be fair, I may very well be wrong in positing this) is that it's a reference to the split rock outcroppings that pretty much surround most of Korovin Island. Using their headings vs current(ish) maps of the region, it looks like they were using sightings of these rock formations for a couple course corrections (which would actually make sense, as using them as landmarks would make it fairly easy to split between Henderson and Korovin, keeping both abeam holding a NExN-ish heading, as they did). Unfortunately, the rock formations on the south side of Korovin appear to have been claimed by the sea (or by man, I'm not entirely sure, given Korovin has some unnaturally straight sections of coastline), but they can be seen quite clearly on the northward side of the island, particularly near the bight on the northward side of the island (a bit WNW of Grosvold Bay). This would also explain the second use of the symbol with regard to the northern bight, as there is a stream coming down from the bluff near the midpoint of the bight.
As an aside, I think it's rather unfortunate that we missed an opportunity when naming Henderson Island. Given that there's already a more famous Henderson Island in the Pitcairns, and the Aleutian Henderson's shape, it would have made for a rather apt Orca (which would be appropriate given the population of Resident and Biggs' in the region). Perhaps if the AAAS gets their way, and a cetacean bill of rights ever becomes a thing, we could see a revisiting of the naming of the Aleutian Henderson Island.