This crucially important caveat they snuck in there:
"Prof Scarborough said: “Cherry-picking data on high-impact, plant-based food or low-impact meat can obscure the clear relationship between animal-based foods and the environment."
...which is an interesting way of saying that lines get blurry depending on the type of meat diet people had and/or the quantity vs the type of plant-based diet people had.
Takeaway from the article shouldn't be meat=bad and vegan=good - the takeaway should be that meat can be an environmentally responsible part of a reasonable diet if done right and that it's also possible for vegan diets to be more environmentally irresponsible.
I keep half a dozen of my own chickens in my backyard...which means about half my daily protein intake comes from eggs (which is a great source, btw). And my chickens free-range in my backyard and largely take care of and feed themselves (supplemented with chicken feed but they get most of their daily intake from the bugs/plants in the yard). I still do eat meat almost daily, but the quantities are a lot less than what I was doing a decade ago, and beef is less than a once-a-week thing for me. Like you, I'm trying to get back in shape and watching macronutrients (like protein) very carefully and trying to hit certain daily minimum numbers.