They are both right:
the cape uses i2c2_scl | P9_19
and i2c2_sda | P9_20
, which is bus I2C2, for the audio codec. However, the pin diagram recommends that you use I2C1 for connecting external devices, and that's on P9.24 and P9.26.
EDIT: I2C2 remains largely free (except address 0x18, used by the codec) if you need to use it for something else