I think he is implying here that it's not hidden. Anyone can see it. Doesn't matter if you are black or white, rich or poor, high born or low born, etc. AKA the dhamma is for everyone to see, just like the moon is for everyone to see.
Our modern, aka scientific, understanding is not capable of even investigating it to begin with.4. I'm not sure about this one, because I haven't looked that much into it. The way the Buddhists saw the geography of their continent, the planet etc. https://en.wikipedia.org/wiki/Jambudvipa#In_Buddhism, is it compatible with our modern understanding?
If it's not accessible to humans, then humans of course can't investigate it and therefore can't prove it or disprove it. If something is worthy of scientific investigation, it has to first be "investigatable" to begin with. Buddhist cosmology is not "investigatable", aka it's unfalsibiable, via modern scientific understanding. If something is unfalsibiable, then modern scientific understanding is not even applicable or relevant.The other three continents of Buddhist accounts around Sumeru are not accessible to humans from Jambudvīpa...Jambudvīpa is the region where the humans live
One could say it's not compatible with our "modern understanding" because our modern understanding is inherently materialistic and Buddhism is not materialistic. A scientist cannot prove or disprove the existence of a deva or deva realm. However, simply because that is the case does not make it false. It doesn't make it true, but it doesn't make it false either. The only legitimate conclusion science can offer is "not applicable" because such things don't even fall within the realm of scientific investigation.