Yeah, a land bridge is still the most likely explanation. But the authors did put this at the end:
“Despite the current evidence, it still seems too farfetched to suggest that H. erectus, or another unknown Pleistocene ancestral candidate for the Kalinga toolmakers (for example, Denisovans), were able to construct some sort of simple watercraft and deliberately cross sea barriers to reach these islands. However, considering evidence of overseas dispersal during the Middle Pleistocene stage is increasing in number, such a hypothesis cannot currently be rejected.” (citations omitted)
