"Clean food" generally means minimally processed--you are eating food that is as close to its natural form as possible, such this[0]. So, meat created in a laboratory is about as far aware from this sort of "clean eating" as possible.
That quoted website just sounds like pseudo-BS-science, especially since the very first thing they mention is removing gluten from diet. That particular horse has been beaten to death and then a bit more.
Our bodies don't give two shits whether the food we eat was grown outside in a natural environment or in a lab - as long as it's chemically the same then there can't be any difference to how we absorb it.
Edit: Ok, after going deeper onto this website it mentions doing juice cleanses and ridding yourself of "toxins". Stay as far away from this website as possible.
Just to be clear, I'm not advocating this website. I'm just trying to point out that "clean" food can mean different things to different people, and that's why some folks might not think meat created in a lab should be considered "clean."
I'm confused, it's not clean because it was grown in a lab, or is there some other factor?