The World Tree was a big secert. But even with Maria's knowledge, at least a hundred people she personally knew were aware of it.
However, they rarely tried to transverse the World Tree when climbing the tower. It's not that they weren't confident in their strength, on the contrary, they were afraid their incredible strength would draw out equally incredible guardian elves.
One thing that became obvious about the World Tree over the years was that there's no way you could win it. You could fight a bunch of elves, kill them all and then go into hiding. Or you could escape from the start and never start a fight with them. But no one ever could continue fighting the elves.
Even the peak rank 6s, on the verge of advancing to rank 7, couldn't achieve such a feat.
There were just too many elves. Even the documented rank 6 elves exceeded 10,000. It made absolute sense for anyone to never get into a prolonged confrontation.