I'm reading it as the DSiWare mode only has the four bonus modes + new mode, and not the core adventure, survival and puzzle modes. I could be wrong but I don't think they would announce a cheaper release of a bigger game before even letting the game sell at its current price?
There is always a perceived fairness issue though. It's tough to go pay $20 for something when you know others can get it for $5 or $10. And yeah I know it's probably cheap on iPhone and such by now but we're talking the same platform here.