no they never promised that.
Please show me a single link to where they said it would be a mix.
They've removed all of the developer blogs from before 2011, or perhaps they never bothered to transfer these blogs from the old site in the first place. In any event, the quote is not on this site, and I'm not sure where I would find it at this point.
There was a dev blog in which one of the developers talked about how creating "only" a theme park was not the right move, and how there needed to be a mix of both theme park and sandbox elements in order for an MMO to have success. It talked about striking the "right balance" between the two, which gave us all -well, myself, anyway- a little hope that BioWare recognized the non sustainability of the purely theme park design model. BioWare abandoned that philosophy sometime in the last year before the release of the game, if it was ever truly a part of their philosophy at all. In hindsight, it seems more likely that they were just blowing smoke.