I think there may be a legitimate tonal (!) reason for the choice of 24 frets, especially if your luthier said that.
There is a node at the 24th fret which divides the string into 4 equal lengths. We know that many guitars intentionally do not have 24 frets, so as to place a pickup at this node point because it happens to sound good to have a pickup there. This is not the case on bass, but on a bass, if we slap an open string, and it hits a 24th fret vs a 21st... I have always wondered if it negatively affects the tone of slapped notes. It makes sense that exactly 24 frets could de-emphasize the fundamental and first harmonics, in a way that more or less frets do not. (Think about how it might emphasize the 12th fret harmonic sound if one were to slap an open string of a 12-fret bass.)
Upon reflection, I have never owned a bass that was not 24 frets. And I have never been great at thumb slapping or getting my slap tone to retain bottom end. Can someone who owns both 21 and 24 frets, and plays slap, speak to their experience with any low end tonal differences?