Actually, garlic was less antilithogenic ("stone dissolving") than onion. That was shown in another study by the same team. I'll post it when I have more time.
If I remember correctly, from most to least antilithogenic was:
1. Heated onion
2. Raw onion
3. Heated garlic
4. Raw garlic
But raw garlic is good in many other ways, of course. And raw garlic was still much better than nothing.