Whatever; After some deeper search into the datasheet, I've found this :
The AXP209 power input can come from lithium battery (BAT),USB VBUS Input, external power
supply (ACIN). The IPS will select an appropriate power source depending on the battery and external
O When only the battery is available, no external power input, the battery powers the system;
O When there is a valid external power source(VBUS Or ACIN), is is the preferred power supply.
O When the external power is removed, the IPS will seamlessly switch over to battery power.
O When both VBUS and ACIN are available, ACIN will be used to power the system and recharge
the lithium battery;
O If the ACIN cannot provide sufficient current, VBUS is also connected to source more
O If the drive capacity is still insufficient, then the charge current is reduced to zero, the
battery is used to power the system
So in the case of the AXP209 IPS, the order is ACIN > VBUS > BAT. When one of it fails, by delivering not enough power or by simply being absent, the IPS chooses a better supply seamlessly.
The answer to the question is yes then, the IPS can be used without ACIN,
Edit : and yes I'm sorry not looking deep enough and answering my dump question by myself