Honestly I can personally attest to vegetables just somehow tasting different for who knows what reason. Back in the day when I was a kid, spinach from the market always taste nice and fresh, but as an adult I noticed there's a difference in the taste even though we've been getting it from the same guy who swears they do it organically.
Some days it taste great just as I remembered, other days it taste bitter and just awful. It's exactly like Oishinbo depicts, and I don't know if that's just me not really caring about the taste of food as a kid, or if something really did change. I can't darn well go to their farm to take a look.
The nutrition issue is something else entirely. Knowing Kariya often fumbles his claims just annoy me, though, I have to just always tell myself to ignore them and enjoy the cooking content.