There's a big gap between that and a vegetarian or vegan diet. There is definitely a lot of room in between for plant-based diets with a reasonable amount of meat (say chicken or fish twice a weak and a Sunday roast once or twice a month). I'm willing to bet that this kind of diet is much more common than one including a hearty portion of meat every day and that comparing this diet to vegetarian or vegan diets would yield a much less significant difference in terms of environmental impact.
That said, I honestly do not think that your diet suggestion is common for Americans, I rarely see people go a meal without meat. The total of 216gm/day is only 7.6 ounces of meat, and I remember getting weird looks ordering only a 6 ounce steak as an adult. And burgers are frequently about a half pound too. Or a chicken breast. It seems totally believable to me that this is average even accounting for less frequent consumers of meat.
I take efforts to rarely eat it so I'm probably barely different from vegetarian. Obviously hybrid diets will fall somewhere in between? I'm not presently an advocate for no-meat product diets because I don't like extremes but I am an advocate for using substitutes when there's little difference. Like taco bell meat could be soy-based meat-substitute ground and literally no one would notice.
Why assume so much? The thesis is in the link you provided. You can easily check whether what you suppose in this comment is true or not.
>> That said, I honestly do not think that your diet suggestion is common for Americans, I rarely see people go a meal without meat.
So this "meat-based" diet is only relevant to Americans? That makes sense- but in that case, the comparison with vegetarian and vegan diets is also only relevant to Americans. i.e. it's American "meat-based" diets that are more environmentally wasteful compared to American "vegetarian" diets etc.