The BATFET is in linear mode when battery voltage falls below the minimum system voltage. The minimum system voltage is maintained by the buck regulator (Q2, Q3, L and C), NOT by the BATFET acting like a boost converter. Fig. 25 should be helpful.
From another point of view, the BATFET will be either OFF or ON if it is not in linear mode. If it is OFF, the battery can't be charged. If it is ON, the system voltage will be dragged below the minimum system voltage. The system can't be powered up immediately.
Table 4 shows the default charging current with different battery voltage. The loss on the BATFET can be calculated accordingly.