I have read that (never mix on headphones).
Personally, I would never tell anyone that as an absolute even though I don't and
if I were asked I would not recommend mixing on phones.
But...that's just me.
The thing that becomes important is learning/knowing how your speakers translate to other systems in hearing every instrument
you want to be heard or predominant.
In other words.....knowing your monitors regardless of perceived quality....which comes with time along with critical listening.
Mixing is quite subjective from my perspective
Don't mean to hijack.....back to topic.