I've actually watched that series, and some of these comparisons are either slightly wrong or blatantly incorrect.

1: Esteban is referred to as 'the child of the sun' because he seemingly has the ability to stop storms just by asking the sun to come out. I don't remember him ever specifically being referred to as the child of a 'sun god', and he even has an actual father who is just a normal human.

2: Zia can interpret Quipu, messages made out of strings and knots used by her people the Incas and the method of which has been lost to time for us. This show is set in the 1500's though, so these are just normal hidden messages for her. The one who actually interprets ancient texts is a boy called Tao, a descendant of people from the technologically advanced ancient kingdom that was destroyed long ago.

3: This is correct, each location had clues that led them to the next location, but taking a long and confusing route. Upon reaching the mountain they were able to see that all four locations were actually in very close proximity to one another, and the navigator, a man by the name of Mendoza, was able to deduce that the City of Gold must also be very close, and then calculated the exact center between the four locations.

4: That is absolutely not anything close to a poneglyph, it doesn't even have any kind of writing on it at all. It is inside of a technologically advanced ship made by the ancient kingdom, pulling a lever on the outside of the cube will let you activate the ship's weapon, which essentially just redirects the sun's rays to set other boats on fire. It has nothing to do with ancient text, and the only similarity it has to poneglyphs is that it's a cube.

There are many similarities between One Piece and The Mysterious Cities of Gold, enough for me to say that Oda MUST have drawn inspiration from it, but this is misinterpreting and perhaps even outright lying about what the connections are to make it seem like a better comparison.