Someone correct me if I'm wrong, but doesn't the Septuagint reference iron chariots somewhere later, just not in that particular verse?
LXE (Brenton) Judges 4:3 "And the children of Israel cried to the Lord, because he had nine hundred chariots of iron; and he mightily oppressed Israel twenty years."
The iron chariots were defeated a bit later:
LXE Judges 4:15: "And the Lord discomfited Sisara, and all his chariots, and all his army, with the edge of the sword before Barac: and Sisara descended from off his chariot, and fled on his feet."
Of the forces of Sisera/Sisara, the song of Deborah relates:
LXE Judges 5:21 "The brook of Kison swept them away, the ancient brook, the brook Kison."
Forces lacking horses and chariots normally would be hopelessly outclassed by forces with them, however Deborah relates a rainstorm so flooded the valley that the enemy soldiers were forced to abandon their chariots and run.
The Heb. text, BTW, can be read as presenting that Judah could not drive the chariots out in Judg 1:9 rather than that God could not; in any case the book of Judges according to all textual trajectories further relates that the forces employing them were ultimately defeated. So the book of Judges hardly seems to intend the task was impossible for God in any case in the final analysis. ;-)