He gained power by going outside and aggressively touching grass and eating a lot of 草
Through the power granted to him enabling him to swallow a bunch of Japanese "LOL", he became all powerful by eating out a plant girl.
And through doing so, it led to an Erofu girl being spared from being Vored.
And him attaining the prized desire of all of the royalties and nobles out there, a relationship with an Erofu.
...
Side note, this really could've been just a straight up fantasy without the need to be an isekai, but sure. It probably just means we'll have a scene about superior Japanese Cooking at some point and Revolutionary Commercial Trade and Production down the line. Maybe even the creation of the Ultimate Melee Weapon, the legendary Katana, even though that was not the actual main weapon used by Samurais back then. Just happy the MC isn't being a wishy washy oh no a nude girl kyaa I mustn't look, my Japanese social standards do not allow me to look at a naked woman's body without incredible internal panic.