By “held their lines” she means they dropped their weapons, abandoned their vehicles, and ran like frightened children.
If there had been any trees in the scramble toward 'retreat". they would still be hiding in them. That is, the ones that already knew they were the correct flavor of izzlamm and wouldn't be summarily slaughtered by IS thugs.
Which brings up another thought. Does anyone else believe that the only arab soldier that will stand and fight IS are the Shi'ites? And the very scarce Christian. All others seem, to me, to be weapon delivery boys who will give up their twice dropped guns.
And it wouldn't surprise me if IS made their captured plant trees along all the highways so their troops could march in the shade.
“The Iraqi security forces have held their lines on the outside of the city,”
By “outside the city,” she means back in Baghdad.