This is true, and you can find this on the McDonald's website. The ingredients list, containing "natural beef flavor," is as follows: "Ingredients: Potatoes, Vegetable Oil (canola Oil, Corn Oil, Soybean Oil, Hydrogenated Soybean Oil, Natural Beef Flavor [wheat And Milk Derivatives]*), Dextrose, Sodium Acid Pyrophosphate (maintain Color), Salt. *natural Beef Flavor Contains Hydrolyzed Wheat And Hydrolyzed Milk As Starting Ingredients."
https://www.mcdonalds.com/us/en-us/product/small-french-fries.html#accordion-c921f9207b-item-283bee7dbd
RATING: True
Something to note is that the term "beef flavoring" is slightly misleading. If natural beef flavor is listed, it doesn't necessarily mean actual meat was used to flavor the fries. In this article, a food chemist explains that: "Food scientists identified the amino acids found in beef, added some very common sugars — starch hydrolysate — put it in a pot, added some citric acid to drop the pH, controlled moisture content, and heated it to the same temperature as meat. Then...*poof* we have meat flavor." The McDonald's fries ingredients only lists "Hydrolyzed Wheat" and "Hydrolyzed Milk" as the starters for the beef flavor, so it is possible that they do not put any actual meat product in their fries. The article you linked slightly glosses over this point, which I thought meaningful enough to reiterate.
https://www.eater.com/2015/9/29/9410199/natural-beef-flavor-vegetarian-what-is-it