That's very misleading. In the late 1970s computers were already 30 years old.
Yes, the "PC" was new. And that's the whole point - we now have cheap (but rubbish) desktop 3D printers. It's the equivalent of the transition from mainframe to minicomputer to PC.
While you're right that both were old technologies. 3DP is noticeably slower in many variables. If we look at the "quality" of output from the mainframes -> Apple II, we see that the calculations would of course be the same and the programming largely the same - the discrete nature of programming being what it is.
However, the dimension of cost for 3DP (which is what the personal 3D printer movement is excited about) is only one dimension of many, and I'd argue not the biggest limitation.
3DP materials and process quality have certainly improved over time, but all of the current technologies are dead-ends in regards to competing with traditional processes - i.e. FDM/SLA/ProJet/PolyJet don't physically seem able, ever, to produce parts as nice as injection molding can for a myriad of reasons - even without cost as a concern. The situation for SLM/ProMetal processes is little better.
Perhaps the recent investment in 3DP will increase R&D spend on quality, but it doesn't seem to be the case if you look at the major or "maker" players.
Yes, the "PC" was new. And that's the whole point - we now have cheap (but rubbish) desktop 3D printers. It's the equivalent of the transition from mainframe to minicomputer to PC.