Yes, insects are meat. The kernel of a nut is called nut meat, even though it is not made from muscle fibers.
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.
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.
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.