The natural pectin in the fruit will aid the thickening. As the mixture boils and the liquid reduces, the fruit will break down and create texture and continue to thicken. If it seems too runny, just let it continue to cook until it's a syrupy, pourable consistency. If you have any leftover compote (unlikely), the flavor just keeps improving. So this is a recipe you can make ahead for a brunch or gathering of any sort.