There’s “diet” and there’s “diet”. When talking about short term, people think of restriction based diets that are kind of framed as challenges “do X for Y weeks to get Z”. The “diet” in this “meme” doesn’t refer to that; It simply refers to how you eat habitually. You’re not going to be healthy if you eat a full size cheesecake every day and vegetables pretty much never, even if you exercise a lot.

That said, most peoples diets aren’t quite as bad as that, whereas it’s extremely common for people to never exercise at all or so rarely that it hardly makes a dent.