I actually don't think the holes were excessively deep or round. Typically they say '$100 hole for a $100 root." You could have added fertilizer, put a couple of feet back in then topped off over the root. Which I guess is what you ended up doing. But digging down that far makes it easier for the roots to grow, especially if you 'laced' the bottom with fertilizer/dirt mixed. The peonies should be great next year.
