When USB (or any 5 V source) is connected to VINx input of the ADP5065 and there is no battery connected, the system output (ISO_Sx) is regulated to 3.3 V typical. Current in no battery -case is being limited by the VINx input current limit. When IIN_EXT = High, the typical input current limit value is 475 mA. Available current at ISO_Sx output depends also on the DCDC efficiency. You can refer to the ADP5065 datasheet page 9, Figures 6 and 7 for an example. Typically at 600 mA output current level the DCDC converter efficiency is close to 90% and you should expect about 640 mA available maximum output current.
When there is no power supply present at VINx, the battery voltage at ISO_Bx is connected directly to ISO_Sx output via battery isolation FET. There is no current limit in the ADP5065 battery isolation FET and system voltage at ISO_Sx is equal to battery voltage (typical Li-Ion could be ranging from 3 V to 4.2 V).
If your system voltage requirement is 3.3 V +/-10%, you will need an external regulator like an LDO.