I'd agree with that, nearly! A fault on one circuit wouldn't affect
other RCBO's unless they were upstream.

No, it could affect an RCBO at the same level.

Consider two SP RCBO circuits in a CU, called A and B.

There is a neutral-earth short on circuit A.

A high current device on A draws some current. A small proportion of the
return current from this goes through the earth fault rather than the
neutral, tripping the RCBO. All well and good, exactly what you want.

However, now a device on circuit B draws some current. As RCBO A has only
cut the live, the neutral current from B could go into the A circuit neutral
and through the same neutral-earth short, causing B to trip also, even
though the earth fault is on a different circuit.

Had A been fitted with a DP RCBO, this neutral would have been cut, so B
wouldn't trip. It is possible that B would trip first whilst A is still
operating. However, once A had tripped or been manually pulled, B would not
repeat the experience.

Christian.