Bach was renowned for his harpsichord playing in France - there is no room for doubt that he played it the way Couperin played and wrote it, in the style of the second image. There are many other historical writings that corroborate this meaning of this text. However, the earlier the music, the less often such slurs were formally noted, even in France. I must stress that this guide applies only to the harpsichord - organ and modern piano require a far more articulated touch than a harpsichord can use. Passages in which successive notes form natural chords benefit from more sustain than melodic passages based on the human voice, which can sound only one note at a time.
Dance music is naturally more articulated than pensive adagios or unmeasured preludes.
Sustaining middle to low thirds, or low fifths, thickens the sound and is only suitable for closes, if even then. Many long passages with no apparent rhythm may be brought to life with rhythmic sustains. The 5th prelude as it may be played Click here to hear it Phrasing of music was considered more important in the 18th century than it has been later on. Sustaining notes is an important component of good phrasing on the harpsichord. The proper amount of sustain is very dependent upon the exact instrument and acoustical surroundings.