The specified power-up time includes the time to fully charge the decoupling cap on the REFIO pin upon power up. The device will not be fully functional before the REFIO pin reaches 1.2V (bandgap voltage). This is a "cold" start.
The power up time is much shorter in a "warm" start as the bandgap voltage is already at 1.2V. A software reset is an example of a "warm" start. It should be less than 1ms depending on the clock rate.