>The kernel of a nut is called nut meat, //
It can be, but it's exceedingly rare IME.
I'm trying to remember but can't quite, there are other non-flesh things someone referred to as meat too, after a similar line to your nut example. It's an analogy, you peel back the skin and underneath is the "flesh" (like a peach, or any fruit).
You could say, when carving a pumpkin, "get right in to the meat of it", but it's euphemistic in this case.
English language is crazy.
I think the line is drawn at poultry and fish sometimes because the input costs are so much lower than mammalian meat. CO2 taxes would incentivize this sort of vegetarianism.