When a chip design is stable enough for manufacturing them. But chips usually have post si bugs and so the tape out is usually a back and forth process depending on how complex thechip is and how competent the design team was.

tldr this article is basically saying the 10nm is so screwed up they can't just adjust a few things, they need to fix a lot and the more changes you start having to make the more verification that needs to be done before things are signed off and ready for tapeout.