- This is Auto-antonymy, correct?
No, because the antonym of "strong" is not "narrow", it is "weak", and the antonym of "narrow" is not "strong", it is "wide".
- Can someone please expound this auto-autonymy?
As in, what kind of a semantic change explains "narrow" > "strong"? Metonymy (in Traugott's sense): a sense which is contextually associated with the main sense takes over as the new meaning. The contextual lens applied to "narrow" is "constriction". If constriction results from deliberate activity (like say someone squeezing or pulling on an aperture to narrow it), then the constriction involves the exercise of strength: the stronger the agent doing the squeezing, the narrower the aperture. The deliberate activity of constriction by someone strong is then metaphorically extended to inanimate narrow apertures.
From https://en.wiktionary.org/wiki/strong, the transition happened in proto-Germanic: PIE *strengʰ- (“pulled tight, straight, constricted”), *strenk- (“taut, stiff, tight”) > Proto-Germanic *strangaz (“tight, strict, straight, strong”) (so *strangaz has been reconstructed as having both meanings).