What source is this? It's clearly complete nonsense even if you have only the smallest understanding of history.
Even if Achilles was a real person he most certainly did not live in 100 AD. Even the Illiad was written way before the first century. By 100 AD, Romans were already a big thing.
I don't know how to say this without coming across as a very mean and big bad person but like
read
I see. I'm still confused about "finishing building the world" thing, where is that from?
Dude, Achilles is a mythological character that the Greeks wrote as living in a time period so far removed from them they only had a vague idea of what it was like. How could you find Achilles' birthdate of all things when the date of the Trojan War is not set in stone?
At the very least, instead of believing whatever random links you find, read the timelines the ancients like Eratosthenes came up with. It's still fanfic but at least it's fanfic from the people who wrote those myths instead of being random modern guesstimates.
This also applies to things unrelated to fiction, but never believe a claim that is without reference and that you can't check yourself.
It's a tricky thing. I wouldn't say those ppl are wrong, because IIRC we don't know for sure the exact location of this "higher dimension" or this "realm of the gods". It can be outside the world like Land of Shadow, it can be within the Reverse side like Avalon and several other places, it can be on another layer of itself. We knew that these gods still watch over mankind in this dimension, and some of them seem to have their own realm like Amaterasu, just that they cannot tamper with the current world that easily (well, unless you're high tier like Amaterasu, Vishnu, Odin).
The problem with the reverse side thing imo, is people counting the gods as phantasmal species who evacuated there. In FGO, because the world texture is destroyed, phantasmal species like dragons and such leaked out from the reverse side and started running around all over the place, but we saw no Divine Spirits running around unless specifically summoned for some purpose in a story chapter.
Talking about this Ishtar also has her own realm, she calls it Inanna
"Only in my company, will you not be a monster"
anywhere than here