That advice is usually more for people who are already fit but don't have abs.

You can maintain a healthy level of fitness by adding some exercise into your life and being mindful of what you eat. But maintaining abs is a whole other project you practically have to center your life around. And indeed, it's mostly a matter of very strict dieting.

Okay then, how about "you can't outrun a bad diet"? Pick your favorite conventional wisdom fitness aphorism as it applies to you, but it's the same underlying principle.