Hi Siebe,
You asked what the meaning of the phrase ‘This is not mine, this I am not, this is not my self’ is.
I see that you have raised this in another post as well. I will try and explain what I understand of this.
Preceding the above phrase (for example in https://suttacentral.net/en/sn22.15) the Buddha says “Bhikkhus, form is impermanent. What is impermanent is suffering. What is suffering is nonself.” In Pali this is “Rūpaṃ, bhikkhave, aniccaṃ. Yadaniccaṃ taṃ dukkhaṃ; yaṃ dukkhaṃ tadanattā”
I understand this as; rupa will not stay the way one likes it to be. If something does not stay the way I like it to be it causes suffering. And if something causes suffering then there is no point in it (it is worthless/pointless).
Then he goes on to say: “What is nonself should be seen as it really is with correct wisdom thus: ‘This is not mine, this I am not, this is not my self.’” In pali “yadanattā taṃ ‘netaṃ mama, nesohamasmi, na meso attā’ti”
I understand this as; If something is pointless / worthless (as it causes suffering), there is no point (it is foolish) to think of it is me, it is mine and something worthwhile to try and make it mine.
Makes sense??
It is also said that fully understanding of the above leads one to become an arahath and that this is the only thing that needs to be done to attain nibbana. So I don’t think that the phrase; ‘This is not mine, this I am not, this is not my self.’ (yadanattā taṃ ‘netaṃ mama, nesohamasmi, na meso attā’ti) specifically refers to sakkayaditti. However, the basic / fundamental understanding of this will lead to the eradication of sakkayaditti. I guess at the ultimate level, the understanding of this leads to the eradication of Maana (asameemana).