LOL! Look, we're not talking about million year old monkey bones here. C-14 dating is generally reliable for anything less than 50,000 years old. I say "generally" because it does vary slightly based on surrounding environmental conditions, and older samples can be off by a couple thousand years (a whisker by the timescales we're discussing). Still, sometimes flukes do happen, which is why generalized statements like "the humans wiped out the sloths" have to be backed up by numerous datings from geographically diverse sites.
In this case, the fossil record is heavy enough to corroborate this theory and offer useful averages. We can tell, for instance, that North America wasn't widely populated by humans 50,000 years ago because the bones dug up so far are almost exclusively newer than that (yes, I know about the flukes, but those could be misreads or even examples of very early explorers). We also know that sloth bones used to be common. We know that the C-14 shows that human remains start showing up around point X, and that sloth remains stop showing up shortly afterward.
It's really not important whether Date X happened 20,000 years ago or 15,000 years ago, but simply that the dating techniques show that the two sets of events happened at the same time.
Look for further developments as Clovis sites are excavated further (they had stopped digging because Clovis was the oldest--in a fine example of circular reasoning) and new finds come to light.
Humans may go back much farther on the continent than previously assumed.