You'd be hard pressed to find anything absolutely toxin-free. Many toxins have non-linear effects, negligible at very low doses, so there are advantages to diversification.

Even if you could find a handful of foods that are absolutely toxin-free, eating only those would greatly reduce the diversity in your diet.

The obvious optimum is a diet minimal in the effect-weighted sum of its toxins, subject to the constraint of also providing healthy levels of all essential nutrients... which the GP is attempting to reasonably approximate without requiring years of research into the exact diet by proposing toxin risk diversification.