«Yaḥūzahā ilā qawlihi» means drawing the report toward one’s own doctrinal position. Al-Nawawī relates it to the Muʿtazilī debate over grave sin and the phrase «not of us».
The hadith alone does not entail definitive exclusion from faith or eternal punishment for every grave sinner. Establishing a text does not validate every interpretation of it, nor an unsupported attribution of it to a particular teacher.
In the Muʿtazilī position described by al-Nawawī, a perpetrator of a grave sin is no longer classified as a believer, yet is not simply labelled a kāfir; he is called a fāsiq and is held to deserve everlasting punishment. Rendering this position as an unqualified declaration of unbelief against every grave sinner misrepresents its own distinction.
By contrast, the affirmative explanation al-Nawawī reports from the scholars for «فَلَيْسَ مِنَّا» here is that such a person is not following the Prophet’s guidance, knowledge, practice and sound manner of conduct. In ordinary usage, distancing someone from one’s way of acting does not necessarily expel him from the religion. Context and the other relevant evidence govern the reading; this is not a blanket rule for every occurrence of the phrase.