Exactly. If the "news team" is already working on stories for 10 o'clock then they just have to write a preliminary version of those stories for 5 o'clock and refresh them for 6. Add in a couple of feature stories to keep the broadcasts distinct, and that's all you need.
If you remember news broadcasts from before the days of CNN, the 5 (or 5:30 more often than not, if there was a broadcast before 6), 6, and 11 o'clock news were all essentially the same newscast. 11 o'clock just had a few updates on developing stories, late sports scores, and the next day's weather. Nowadays there's more distinction, but not all that much.
For the expense of maybe one or two more segment writers for features exclusive to the 5 and 6 o'clock broadcasts, a station can put on an early newscast with more avails than most syndicated shows at a much cheaper rate and with no barter. And they can even afford to sell those avails at a lower rate and make a better profit. Why do you think the ABC O&O's decided to replace Oprah with a 4:00 newscast?