Pretty sure the answer is yes.
DUAL LINE mode is how I typically use this recorder, yet I always have an external preamp running into both the mic and line inputs. However, if I was to unplug the external mic input I'm pretty sure the recorder reverts to the internal mics.
Keep in mind that you can turn on/off PIP to the mic input in the menu. I'm not certain, but I don't think that setting affects the internal mics.