I made a quick check on my 1975 F50R , 1977 F512 and 1992 X-2000 - they all have mop inlay blocks w abalone triangles - and they all are the same - the tip of the triangle is NOT halfway between 3rd & 4th string. You can also see this in Hans book cover.
I measured some blocks to verify, that they indeed were dead center on fretboard.
Also my 1957 baby-Gibson has dot markers closer to D than G string.
It looks like the outer edge of the string is equally close to fretboard edge - and the space between strings is equal - naturally this means that the space between 3-4 is not in center of fingerboard. Because strings are thicker on one side.
If we were to cut nut slots at even intervals center-to-center and do the same at bridge - then cosmetics would be better - but fingering might first feel odd - at least for those who struggle w that 1/32 narrower nut.
Fortunately none of this has ever prevented my joy of having these guitars