Not gonna lie, this article is pretty stupid.
As others echoed, I rarely ever eat grains, and when I do, in very small amounts.
Furthermore, the author's assumption that arable land would have to be more intensely farmed, increasing use of fertilizers, pesticides, chemicals, etc - is somewhat false. I guess if you are into chemically intensive agriculture, yes, but I'm certainly not. Even today, we are uncovering more and more gardening methods that are less intensive - both chemically and in terms of physical labor.
Pre-WWII, Americans grew 50% of their own vegetables. If everyone had a kitchen garden (and maybe some hens and a couple of dairy/meat goats) then chemically-intensive agriculture would decrease by a significant amount. Organic and sustainable farming can indeed feed the world, as a UN report concluded.