Researchers have linked dietary data from over 55,000 individuals with data on the environmental impacts of the foods they eat. The team, from the Livestock, Environment and People (LEAP) project at the University of Oxford, found that the dietary impacts of vegans were around a third of those of high meat eaters. They also saw a 30% difference between high- and low-meat diets for most of the measures of environmental harm.
Notice the may.
Eating animal foods is also linked to increased risk of certain types of cancer, stroke, etc. It’s totally possible to avoid malnutrition even on a vegan diet. It just requires some consideration.