Theres a long article on Hono Floriensis, in New Scientist, behind a paywall. Ill paste the gist of it here
An early human presence on Flores seemed particularly improbable because of the island’s weird biology. During the 19th century, biologist Alfred Russel Wallace had noted that the islands to the north and west of Flores – including Borneo, Sumatra, Java and Bali – are typically inhabited by Eurasian species like orangutans and tigers. They are largely absent to the south and east, on Flores and other islands including Sulawesi, Lombok and Sumbawa. Eventually biologists came up with an explanation. In prehistory, sea levels were so much lower that Borneo, Sumatra, Java and Bali were part of the mainland, but a deep marine trench to the south-east, swirling with strong currents, kept Sulawesi, Flores and their neighbours separate. In other words, snaking through the islands of South-East Asia is an invisible species barrier, now dubbed the Wallace line, that apes and tigers could not have walked or swum across. In places it is narrow: the strait between Bali and Lombok is little more than 20 kilometres across, but remains impassable to large species."
So what they are saying is How did homo Floriensis get onto that island, if it has neve rbeen possible to cross it by walking or swimming?
"In the 20th century, it was generally agreed that Eurasian megafauna – species the size of humans or larger – had breached the Wallace line perhaps just twice. First across were elephants, powerful swimmers whose relatives left fossil remains on Flores and elsewhere. Then came H. sapiens, who must have breached the Wallace line en route to Australia around 65,000 years ago."
So if they couldnt swim or walk there, how did they get there.?
"We know, for instance, that small monkeys somehow made it across the line to Sulawesi, and clearly they didn’t use boats – in all likelihood they floated over on mats of vegetation. Gerrit van den Bergh at the University of Wollongong in Australia has found stone tools on Sulawesi that are at least 100,000 years old, and thinks this is where the first Floresians came from. Just last month a team announced they had found 700,000-year-old stone tools in the Philippines, offering another possible route."
"Then there is what has been dubbed the Robinson Crusoe problem. A single individual surviving on an accidental raft couldn’t possibly establish a viable population on a new shore. “To be archaeologically visible, you need thousands of people,” says Bednarik. Even if small animals like monkeys crossed the Wallace line accidentally, it does not follow that humans did so too. And although an accidental raft could carry dozens of small monkeys, it is hard to imagine one able to transport hundreds of people."
"The conclusion? Early humans wouldn’t have cobbled rafts together on impulse. Building a seaworthy vessel took months of planning, suggesting it was impossible without advanced communication skills. Indeed, earlier this year, the linguist Daniel Everett at Bentley University in Massachusetts used the controversial evidence for ancient seafaring to argue that species like H. erectus must have had abstract language. What’s more, Bednarik’s voyages show that failure, and potentially death, would have been close companions of the ancient mariners’ attempts. He thinks they would have needed persuasive arguments to convince others to commit to such journeys."
SO ive cut a lot out, the article is too big to copypasta the lot, but the essential conclusions is the they must have crossed deliberately, and to do so would have required a large group effort, and maybe many trips to get enough people over to sustain the population. So that meens they had an advanced language which enabled them to debate and discuss the idea, and agree on an abstract concept of crossing the straits, and that they had tools and advanced boat/raft building skills, 65,000 years ago.