> According to the documentation the voltage regulator can be disabled by connecting the EN pin to the ground. It looks however that this cuts off power to the processor, am i rigth ?
Correct, at least thats the case for the Adafruit boards, yours could be different I guess.
> Would it be possible to power the board then by connecting a lipo directly to the 3.3V pin ?
If you try that you stand a very good chance of destroying the board, LoRa device etc. A LiPo could be up to 4.2V, not a good idea to connect one to a 3.3V circuit.
> If not what is the reason of being able to disable to voltage regulator ?
So that something external can turn of the 3.3V output of the regulator, and hence also the components connected to it.