The Civilization_FreeUnits list is for units you get at the start of the game, regardless of anything else. But if you want to know the units given for a late-era start, that's something totally different. That is, if you start in the Industrial, then you'll start with a few Riflemen, a couple Workers, and a couple Settlers, and that's all handled through the Eras table.
The way it does these is using the <StartingDefenseUnits>, <StartingWorkerUnits>, and <StartingExploreUnits> tabs.
"Defense" in this case will be the highest-strength resourceless combat Land unit available. Generally this'll be an infantry-type unit.
"Worker" will be the most recent unit that can execute Build actions, so if you've made an upgraded Worker unit in later eras (like I have) then it'll give that instead, but I'm not sure exactly where the threshold is. If you start adding Legion-type combat units that can build things, then I'm not sure whether it'd start giving that unit instead. (But again, the unit has to be resource-free.)
"Explore" means Settlers, which I know isn't obvious, but it's what they did. This'll be the highest-tech resourceless unit that has the <Found> tag that lets you found a city, so if you add an improved Settler to the game, it will start using those instead if you start the game in an era beyond when they unlock.