As always, it's a matter of questioning the words used in the question; specifically -
What does it mean by "teaching"?
What does it mean by "formal"?
What does it mean by "grammar"?
At one end of the spectrum, the phrase 'formal grammar teaching' means encouraging students to think/discuss/internalize principles of how bits of language fit together, and at the other end the phrase means indoctrinating students with a set of prescriptive "rules". Loads of room for discussion there. No time for me to jump in though.