I remember hearing about the Beagleboard for years before the Pi, along with the Plug machines and Gumstix (if I'm remembering correctly; never really dug into those at all). I think the RPi's greatest innovations have been their educational goals and (crucially) the price point. I think that most of the boards before it were marketed as hacker/dev boards and priced accordingly, and they were correspondingly more niche products.

It certainly wasn't the first hackable SBC, but I think adding "mass-market" into the claim makes it defensible. There's a parallel to the Arduino, in my mind. It wasn't the first microcontroller dev board, but it was the one that made that microcontrollers look cool to outsiders.