Ah, I thought you were talking about the area for the loops. The idea behind this is because running conductors through the substrate has its own issues that i

Ah, I thought you were talking about the area for the loops. The idea behind this is because running conductors through the substrate has its own issues that ironically makes it a lot more complicated than little transformers like this. It's all material science kind of stuff about how the extra steps to bond the layers conductively runs into yield issues. They also thermally flex in weird ways reminiscent of 2007 era BGAs that had problems with the shift to ROHS detaching themselves from boards. But way more fragile and you can't fix it by a run through the reflow oven. Obviously they've gotten past some of those issues in the past few years.