This is a legitimate line of research, but the headline is misleading.
For the most part all nations can and are feeding themselves. It's saying they can't produce a balanced diet domestically, they rely on trade.
It would be like saying "Most people can't attain clean drinking water." Sure, I can't install plumbing myself, and I don't have a water treatment plant. But I do have clean drinking water. I pay the town for the supply, and I pay a plumber to install pipes in my house.
What the study is highlighting is that in the event of a disruption to trade, like a war or new nationalist policies, pretty much all countries could face difficulties. Of course countries and people make changes to their behavior based on changes in circumstance, and a balanced diet is different than a life or death diet.