Schnabel is indicating that the bass voice here should be given more prominence than "just accompaniment"; rather, it should be played with the quality of a counter melody. He makes a point of marking all four notes that he deems of "melodic" importance.
Goldenweiser indicates a similar interpretation, but does so by means of a single, longer diagonal, which is a typical notation for "the melody moves from one voice (or staff) to another". (See also: Piano Sheet Music Symbols of Long Diagonal Lines
Goldenweiser makes explicit that the
C-B-Bb in the treble staff is a musical gesture continuing in the bass staff, but he doesn't indicate how long that gesture continues. Schnabel takes the gestural transition from treble to bass staves for granted, but explicitly marks the notes comprising the gesture.
Barry Cooper, in his ABRSM edition, offers a third notation for the same ideas, using a bracket to suggest that the lower notes in the treble staff (the
Bb) be played with the left hand to facilitate the line's continuation into the bass staff. (Regarding the bracket, see What does the L-shaped symbol attached to C5 and G4 on the top staff mean?