Then... and I may be stretching the theory a bit, here it says that the striped thing on his chin is the rest of the mushroom. And it kind looks like Usopp's striped armband. I know he sometimes take it off, or it up, so it's not part of his arm. But what if he just kept that part?

And then we have this moment, when Nami had a fever.

We know Luffy ate the fruit, and we know Sanji is genetically modified, so what's Usopp deal then? It doesn't feel like a lie or an exaggeration, he is saying it at the exact same moment Sanji is.