The claim here is that carnism is necessary to avoid protein deficiency
As shown in debunking the myth that vegan food is expensive, protein isn't any less available on a vegan versus a carnist diet. This argument stems from people associating the word "protein" with "meat",. In reality foods like ground beef are a very poor source of protein, and are beaten in cost per gram of protein and protein per calorie of foods like tofu and lentils. It actually also loses in a head-to-head match against most vegan ground beef crumbles, the antibiotics and hormones notwithstanding.
Given that an average 38-year-old male, 5'10", 160 lbs needs 2250 calories and 60 or so grams (depending on whom you ask) of protein, one can get their entire protein requirements from eating only potatoes in a day (Same with, for example, an average female of 5'7" and 150 lbs). It should also be noted that the DIAAS score for potatoes is 100% - making them the highest quality protein you can get.
By contrast, even though most people consistently demolish their protein requirements, only 5% of people are meeting their daily fiber requirements. Eat a vegetable if you're concerned about deficiencies.