My guess: Only people who have really good mana-handling abilities can discern the "true" taste of what he's cooking - maybe if you're able (or have very high potential) to efficiently manage mana, maybe it "harmonizes" all the flavours properly?
I'm pretty pleasantly surprised - I was expecting the dude to be a total dickbag - but aside from being a bit opportunistic he's actually pretty decent? Like, he didn't even consider running away.