Nasal breathing

I first read The New Alpinism a few years back and so until more recently had been under the impression that I could use nasal breathing as an indicator of still being inside the aerobic base training zones. However I now know that this apparently isn’t a good indicator for untrained athletes- but could someone elaborate why (in which direction doesn’t it work if consistent?) and at what point in one’s training progression might it become a good indicator?

I started getting curious cause as I got better at nasal breathing I noticed I could continue to nasal breath upward of 80% of my max hr