Simply preparing food at home with fresher ingredients and being conscious of what you're taking in can account for much of the gains they experienced.
The vast majority of people on the planet don't eat 90% meat diets, but if we did it would be a disaster! I'm not saying it isn't the case with you, but anyone who experienced amazing gains shifting their diet to mostly meat should consider survivorship bias, and that the benefits are from cutting other things out, not adding tons of animal protein in.
This was the clincher for me. I listened to the audiobook "How Not to Die" by Dr. Greger and was convinced of the common sense that eating plant based whole foods is the way to be healthier. The lowered cost and lowered impact on the environment are just icing on the cake. If eating meat was better for me, I'd still do it. The science done so far seems to suggest that a whole food plant based diet is the healthiest diet.