It's not a mistake. One of the definitions of "determine" according to Merriam-Webster is "to find out or come to a decision about by investigation, reasoning, or calculation." For example: How do you determine right from wrong? After some discussion by the judges, it was determined that the red team won the match. Whether someone is food insecure is determined by whether he/she has regular access to adequate food. That's the definition used here. Maybe it's not the best word choice since apparently it causes confusion, but it's not wrong.

I agree with Wei, Mikkel, & Dan (Brenda and Teacher Jan are just incorrect in this instance). This is a very academic usage of the word "determined." You probably will never hear it casually, but you'll read it a lot in papers, studies, and journals. It means that someone examined evidence and made a decision. If I said "I was able to determine that this is an apple." It means that I looked carefully at the fruit in front of me, saw that it was red, recognized the smell, and came to the conclusion that it was an apple. In your case "and 27.4 million people in the region were determined to be food insecure" means that someone (it is unclear who, because this is passive voice), looked at some data about a large group of people. After doing that, they were able to decide that 27.4 million of them were food insecure.

There is more than one meaning of the word "determined." The common meaning of "determined" means firm, resolute, even stubborn. "I am determined to solve this crossword puzzle no matter how long it takes." Another meaning is "made an official, authoritative judgement about whether something is true." It is often used in this sense in law and politics. Here are some possible examples. "Normally, you can't build a house that big on a lot that size, unless it is determined that the extra space is needed to accommodate a person with disabilities." "Who makes that determination?" "The Zoning Board of Appeals." "The Scholarship Committee has examined your application and has determined that you meet the eligibility criteria for the Lydia Cramer Memorial Scholarship. Your next step is to collect and submit the required materials, including the letters of recommendation..." A famous warning that used to appear on cigarette packages said "Warning: The Surgeon General Has Determined That Cigarette Smoking Is Dangerous to Your Health." In some cases, a "determination" means that something would be accepted as true in a court of law. In your case, "27.4 million people in the region were determined to be food insecure" means there is some official, formal report, using some clear, recognized definition of what it means to be "food insecure," that led to that number.

"Millions of people in this region were determined to be food insecure." I can see your confusion. This sentence doesn't make sense. Why would millions of people decide not to have food security? The opposite would make more sense: Millions of people in this region were determined to be food secure. (The synonyms I would use here are 'resolute' or 'decisive'.)

Determined here means they set their mind to something, they were single minded that they were not going to have enough food, and nothing was going to change that fact. They made up their mind about the situation.