It’s because of the taurine that they must be. If they can get a supply of taurine, they might be fine. I remember reading about a lion that wouldn’t eat meat, but he was fine because they made sure he got his taurine. I don’t know if he lived as long or was as vibrant as he should have been.
We are constantly finding out new things which change our previous views. Consider baby formula: when they started it in the 1920s, they didn’t know about a gazillion things that are in breast milk. We learned more, improved the formula, and still babies do better if they are breastfed.
The inclusion of actual stuff the pets eat, such as meat and fish in cat and dog food, sanahorias para los conejos, etc. will probably be shown to be better in the long run. I notice the study did not say they took differences in environment, etc., into account.