I'll have to look up that idea of the "nano assembler" that you're referring, but I'll just mention that there is significant additive manufacturing in a modern semiconductor GAAFET node, with the entire transistor being grown.

This video is a good summary of the main steps involved: https://youtu.be/xaKyDrWfHes

It's certainly not as powerful or universal as 3D printing and it might not even be "additive manufacturing" proper, but it is a lot more than just etching/subtractive manufacturing.