I don't see how this is a valid conclusion. Not having a sales tax does mean that the poorest people are not hit as hard, but it also means that the wealthiest folks, who make much of their income from capital gains, get by paying less than they probably should. Our property tax system unfairly hits newer homes harder than older homes because they don't receive the assessed value break that the older properties get. Often this means that people living in houses that have the same market value pay widely different property taxes. It's always pissed me off that property taxes hit the people who choose to put their money into the homes they buy harder than the people who choose to live in more economical homes but spend their income on toys or travel. And it especially bothers me that older people don't get any property tax relief and often have to downsize their homes when they retire even.
I guess their is no perfect system, but I think these guys may have not weighted some of the disadvantages of Oregon's system the way I would have.