That passage in Ghosts of Onyx is wrong in itself - if a child was old enough to remember what happened on the refugee ship (say, five years old in 2525), they would be 12 by the time the S-III program started training the kids (2532). Also, Jericho VII didn't get glassed until 3 years after that part of tbe book.
Look at page 96 of Fall of Reach - by the time the first UNSC ships got there, the ones that got destroyed in orbit, the planet was "a cratered, barren desert. Thin grey sunlight reflected off a glassy crust. Heat wavered from the surface. Isolated regions glowed red."
The only time ground forces could be on Harvest, unless they were already there when the Covenant attacked (pretty unlikely) is when Cole arrived months later - and the Covenant, according to the books, had already left the system then and moved to other colonies.
