Some people point out that Egypt and Rome did overlap. YES, I know this. However, just because you've personally logically concluded that the Egyptians could not have been there prior to the events we saw from 2000 years ago, doesn't change the fact that the initial implication is in fact just that.
If you think that the egyptian influence came AFTER that, then we are talking about Egyptphiles, NOT dynasties... e.g. perhaps Jacob just likes it and had some losties build his huge statue as his home. Before I address that let me reiterate this is something I don't think they will answer in the show, this is background, and it is for us to figure out likely acceptable interpretations, and hope there are not fatal contradictions to each interpretation.
Someone suggests perhaps egyptian slaves were brought to the island at one time, the culture of those symbols was still alive (again, it's still alive now, perhaps Henry Gale was an egyptologist... <sarcasm>)... but it takes a lot of people and supplies to build such a thing. And though it could be built by men arriving on ships in short order, to be able to get to and leave the island and build a statue taking a hundred men (?) shows mastery of the island.
Do you really think it's likely that after the events in Across the Sea, Jacob allowed such a culture to live on the island, master it's secrets? For anyone to have gotten that close to the truth about the island, it must have happened before Across the Sea.
And to be clear, I'm not saying MiB discovered the caves, I think he built them by manipulating castaways, but I think the Egyptians had been there before as well, and could easilly have left hieroglyphic explanations of their properties and even how to use them. Yes the show doesn't say this, but it would explain how romans could get ideas about how to use the information.
PS: I will add I find "channeling water and light" a perfectly acceptable description... so what if it's EM or quantum laws, yes, you can affect EM with water, and bend and manipulate light. I wouldn't expect the ancient explanation to match the modern, and I expect it to be a product of thousands of years of trial and error rather than drawn from abstract principles like the standard model.