This image has to be wrong then?
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That picture is of polarized capacitors. Those devices will
typically fail catastrophically if a reversed voltage is applied.
The markings on a polarized caps have a different meaning
than the markings on diodes.
For the purpose of describing how diodes are used in a
keyboard switch matrix, I think that '+' and '-' can only be
confusing, inappropriate labels for diode terminals.
At various points during the scanning of a matrix with a
pressed switch a diode will have voltage across it in both
directions. Anode and Cathode are the correct terms/labels.
The cathode is marked, typically, but always check your
datasheet, then check how they are supposed to be used
in your particular circuit.