i think the right answer is that the washing machines we have now are a lot better than the ones in our parents' day so they are capable of handling the poopy diapers without the extra scrubbing and soaking.
I'm sure our hot water heaters are more effective than our older generation of clothdiaperers. Also I'm sure our detergents are more effective at cleaning than they used to be. The hot water kills the e-coli. It would be interesting to do a culture of our clean diapers to see if they grow anything, though. If anyone has access to a lab please do this and let us know!