The hard to believe part would make it an effective switcheroo reveal.
It would be an effective switcheroo because introducing it now without any foreshadowing after 1000 chapters of Luffy using it is a huge retcon.
As for why nobody would know it's cause the real name was purged from history. Like I'm not quite on the same page with people assuming the elders just would know everything given the scene to me reads in a way that clearly highlights them not knowing the exact history behind that fruit with the youngest looking elder asking the questions and the older ones stating it is even a legend to them now.
I don't expect many people to know and also not to the fullest extent but some of the Five Elders clearly know at least the basics. We've seen all five of them react several times whenever Luffy got into trouble and none of them commented on his powers. Them now revealing that his abilities might actually be the erased fruit would have absolutely no setup when there were plenty of opportunities to do so. I trust Oda to do better.
That the fruit is publicly known doesn't matter when its real name was purged and it got renamed as long as the powers match the new name.
Its real name being erased from history doesn't explain why the people who at the very least know of this legendary fruit never made the connection. What would that look like? "The abilities this troublesome rookie has demonstrated are very similar to that fruit that's supposed to be a myth. But it's obviously not that one because Straw Hat Luffy clearly shouts 'gomu gomu' before his attacks, so it's the Gomu Gomu no Mi. Crisis averted."
The way I think of it is what if it was the heat fruit of Oven, the whole world thinks the fruit's power is just heating up things but in actuality it's the whole xmen thing with speeding up molecules is its real power and heating up matter is just a small part of its real power.
Okay, let's use your example. Let's assume that there's a legendary fruit that can speed up molecules. That ability is then known to some people who try to hide it behind a different name. They come up with the name "Heat Heat Fruit". After centuries this legend is only known to two people, who can recognize the ability and know that its current name is just a disguise. Then some teenager starts to cause trouble who can heat things up. Would that not raise suspicion? Why would they connect the dots only two years later?
In my mind the reveal playing out to something similar like this would make it the most effective. As for why the gum gum fruit well we have this dangling plot thread on why in the seven hells Shanks was going for this fruit, we have the thread of Shank visiting the gorosei to talk with them about a certain pirate and when we look at the Kaido vs Luffy fight the next logical power up might be Luffy's awakening. In that sense there are some things that would line up neatly for a the gum gum fruit was the "whatever has the properties of gum but also these other powers" fruit.
This is a perfect example of the human brain obsessively trying to make sense of things, even when they might not fit together. Shanks going after the Gomu Gomu no Mi might just be as simple as a pirate going after a government ship for loot, Devil Fruits are highly sought after no matter what ability they give. Luffy already got two power-ups this arc and is currently evenly matched with Kaido. Things only line up neatly if you ignore that all the minuscule setup for it has been done in the last 100 chapters while throwing out the 900 that came before.