Hey Mark. Good question and one that I have toiled with over the years. I neither support nor refute this implied definition. But, it seems to have to do with whether you register the car for your own personal use or perhaps it has to do with the length of time you own it. Even our own, Colin, uses this term on some of his cars - see this
'71 Plymouth 'cuda 340 or this
'70 Boss 302 Mustang currently for sale. For me it is all about due diligence. Just as you scrutinize the "numbers matching" or "original drivetrain" or "original buildsheet", or "documented owner history", you would investigate the "one owner" aspect as well. If you, no kidding, want to be the one that buys the car off of the fellow or gal who originally bought the car back in 197X, it should be obvious to you that, in the case of Colin's 'cuda, that he was either not born or just a wee little tike.