Many people can't make enough carnitine and need it from meat
Healthy people make all the carnitine they need in the liver and kidneys, and the kidneys hold on to most of what is made. Vegans tend to have somewhat lower blood levels but not deficiency. The rare people who truly can't make or keep enough are treated with a supplement that doesn't come from animals.
Carnitine counts as non-essential only on the assumption that everyone makes enough of it, and people differ. Vegans have measurably lower carnitine levels than meat-eaters, and a meaningful share of the population may not make enough to make up the gap. For them, food sources matter, and those sources are almost entirely animal foods. A diet that depends on everyone's body working well is a riskier bet than it looks.
There is no good evidence that anything like 30% of people fail to make enough carnitine. It isn't classed as an essential nutrient for healthy adults or children because the body makes it from two amino acids, lysine and methionine, with help from vitamin C, B6, niacin and iron. The kidneys also reabsorb almost all of the carnitine that passes through them, so the body holds on to its supply. I'm not aware of any recognised source for the 30% figure.
Studies of vegetarians and vegans do find somewhat lower carnitine in the blood than in meat-eaters. That part of the claim is real. But the levels usually sit within the normal range, and no clinical deficiency has been shown in healthy people eating plant-based diets. Lower intake mostly means the body leans more on making its own and losing less.
True carnitine deficiency exists. The primary form is a genetic disorder affecting roughly one in tens of thousands of births. Secondary forms come with kidney dialysis, some medicines such as valproate, and premature birth. These are medical conditions, and they are treated with L-carnitine, which is made synthetically rather than taken from animals. People with these conditions get more reliable treatment from a measured dose than from eating more meat.
So the claim names a real nutrient and a real but rare medical problem, then attaches a figure that the evidence doesn't support. Even where someone does need more carnitine, a supplement covers it, as with B12. That doesn't make meat a dietary requirement.