I'm currently reading "Oak Island Mystery: Solved: The Final Chapter", her 2019 book. It's the most convincing theory, but it has no good explanation of Nolan's Cross. Even if you assume the island was clear-cut back in the day to harvest ship masts and stoke the tar pits (a likely case), and such a cross would be visible to ships from the ocean, a cross of that form still doesn't fit easily into the story. Natural places of worship would be an amphitheater-style ground profile. And instinctive construction of giant ground level art would take the form of mounds, like the templar serpent mound. Not a connnect-the-dots construct that no one but a surveyor could see.
Although they did presumably have access to slave labor at some point, the ox nails indicate Nolan's Cross may have been constructed by an earlier crew using Oxen rather than slaves. But the ox nails seem to rule out alien hands.