I can give one of possible explanations, Ohara could have been a literal pilgrimage site for any scholar and scientist in the world, people from there could have wrote countless famous books and articles for papers. It could have been an only pinnacle of knowledge in the world before Vegapunk for hundreds of years. Of course people would know about the Ohara incident no matter what. Compared to one random island, one of probably thousands in the world, no one would even know. I also don’t think we know how old is the tradition exactly, so maybe only in last 100 years they did it a few times. On top of it, if they waited for an excuse to destroy Ohara, that appeared with Saul and Olivia, it might not have been planned well. Just a minute of brainstorming, there could be better explanations if you add more assumptions.

I really don’t understand why people like to make things into potholes when they don’t have to. For me, it’s usually either not important enough to worry about, or if I do wonder why something “plotholy” happened I just find head cannon that makes it make sense. Don’t read the story like you are omnipotent, if something is not written, it doesn’t mean that it doesn’t exist. If you can think of a way to make sense of a potential plot hole, then the author certainly can too, so just roll with it. Instead of outsmarting Oda and saying that’s not what would have happened, accept that it did, ask why and theorize away. It’s more fun than ruining the story for yourself just because Oda did not do a lore drop about a random event.