Current Consumption of CANbus (CANH and CANL)
The max supply current of 17mA is with "no load". That means there is nothing on the output to supply output current into. Any output current, up to the limits in Table 8.5, is in addition to this rating.

This approach is often used in datasheets because it gives you the values of current "consumed" by the device. Output current is consumed by external loads (in this case, the bus) so is useful to treat separately. The necessary output current is very dependant on the external environment, rather than the behaviour of the IC itself. Indeed, often these driver ICs have a separate supply pin just for the load. While this one does not, you can still see from the block diagram that the output could have its own rail.