Iunno. The mythology around the admirals (and, potentially, Monkey D. Dragon) is obscure enough that I don't even remember the details; something about "Four Great Seas," various animals, etc. It's more or less described in here, starting at about 2:05 (though I think the whole video is worth the time, just for entertainment value; all the stuff about ancient weapons is purely speculation, and doesn't matter for our conversation here lol).
https://www.youtube.com/watch?v=NKXPhEPpPkU
Even if you disagree with the conclusions, it does seem like the admirals are based on a fairly obscure piece of Chinese mythology. Maybe it's just obscure to me as an American, and Japanese 12 year olds would be familiar with it... But I have no idea if that's the case.
My only point is, sometimes, Oda does use seemingly obscure mythology. I agree with you that Typhon is a stretch, but only because there is a lack of evidence, not because Oda doesn't use obscure mythology.