This is the crux of your argument. We cannot simply assume someone has experience they have never shown on screen. We apply
Occam's Razor to evidence here on the site; which means the less contrived argument always takes precedence. Assuming someone has experienced magnitude 6 earthquakes with no shown or spoken evidence is more contrived than assuming otherwise, hence we go for the latter.
We simply do not have enough evidence to support any magnitude of the earthquake. There are so many factors to take into account like where the source of the earthquake is, how far away from it was Yujiro, how long was it going on before it reached Yujiro, etc. These are all things we need to take into account and just can't because we lack the evidence from the manga itself.
This is true, yes. Yujiro is leagues above Oliva. But is Oliva has shown feats far above Yujiro's best-shown feats, we do not assume Yujiro scales above those feats without reasonable cause. And as I have explained above, no one has reasonable cause to assume Yujiro is an island buster.