650 mW power dissipation is most likely correlated with the "dominant state" supply current of 232 mA (the actual supply current required on the isolated side is less than half this). Technically that's more of a fault condition. In fact the 1 Mbps supply current case is really a worst case, compared to normal operation, as it's a constant 500 kHz clock input. Unless the transceiver is retransmitting a CAN frame of mostly dominant bits, at the worst case retransmit (e.g. no responding CAN nodes), supply current in operation will undoubtedly be lower.
Regarding thermal information, the absolute maximum ratings provide that the junction temperature must remain below 130C, and there is a Theta Ja value of 53C/W provided as well. As the maximum ambient temperature is 85C, using the formula Tjmax = Ta + ThetaJa * Power Dissipation - we can see that this allows for the power dissipation experienced in the fault condition (stuck in dominant state).