Allow me to weigh in.
Lord knows I read far too much about chickens, eggs and health and I am not a fan of blanket statements such as "its a myth that backyard flocks don't have salmonella", without siting where this information came from. That said I subscribe to Back yard poultry magazine and after the recent outbreak of salmonella in the news, the author of an article had her birds tested and found she did not have salmonella in her flock. Unfortunately I can not keep every issue or I'd have no room for chickens. Lord have mercy! So I can not site the issue though it would have been in one of the last two. One thing is certain, salmonella can be prevented or at least the risk held to a minimum by proper husbandry.
Now as far as washing eggs, I suggest an experiment, in a setting of washed and unwashed eggs. MY last hatch yielded 25/25 chicks. 5 of those eggs I had washed and only 3 of those had viable embryos on day 8. I can't say how many of those hatched due to the fact that I took them out of the turner and they were lost in the shuffle. 4 of these 25 chicks that hatched did have to be culled for problems. Whether it was due to heredity or something else I don't know. There are so many variables to hatching it would take a whole library to hold all the research needed to say what and why eggs don't hatch. The recent issue of Practical poultry states that age can decrease the hatchabilty rate in hens older than three years. Since I have 4 of such hens it may have been that or the washed eggs, or the gene pool or, or, or maybe the momma hen didn't show them enough love. Sorry please excuse my sense of humor.
Now as far as hatcheries go, videos I have seen show the attendant pulling out a drawer and there seem to be alot of unhatched eggs. Knowing the business end of things, I know that hatcheries can afford to set millions of eggs and toss alot of unhatched eggs and still make a profit. My point being yes they wash their eggs but that doesn't mean they are getting better hatches than you are at home. I am basing, of course my opinion, on videos I have seen, not specific research. I had one hen hatch 2/2 this time last year and hatch in the dead of winter, below zero temps. Either she never got off to eat and poop or there was a secret heater I couldn't see in the nest box and I did find pizza delivery boxes in the coop. (sorry) I did feed her bits of bread on occasion. (There are some who don't think it's ok to give chickens bread btw).
Finally just FYI a recent reading of the Alberta site, gave information about an outbreak of AI in a hatchery in the Manitoba area and possibly more than one. This resulted in a government conducted and overseeing of a thorough cleaning and disinfecting of the hatchery. It seems to me whether it is beneficial or not to wash eggs hatched in an incubator is entirely an personal one, though I have never witnessed any of my broody hens washing her eggs before commencing to brood them. When I do maybe then I consider washing her eggs too.
Wishing us all successful hatches whether washed or unwashed.
Rancher