The best processor for any given project is...
...the one you already know inside out. The hardware, the firmware, the dev environment, the bugs etc. If the designer knows the 8051 then moving to an ARM core would have cost money and time. I wholeheartedly agree though that Elektor should move to open source their software. This can only benefit the product in the long term, as people debug it, add extra functions, improve accuracy (I'm preaching to the choir here).
I would have used an AD5933 anyway (demo board $59 from ADI) which has its own flexible DDS frequency source, current-to-voltage conversion, 12-bit ADC and even an internal temperature sensor. I say 'would have', I did. As ever though, without the motivation of commercial product deadlines, it died a fuzzy death somewhere after I laid out the first PCB.