And they know this because...
It is just the 1st one they’ve found, but I’m sure they will find even older ones-slavery has been around for at least as long as humans have been recording events by drawing/carving on rocks. There are plenty of drawings of slaves in shackles/ropes being led around/working, as well as talking about it in the Torah, Bible, and numerous other accounts. I’d bet that our Neanderthal ancestors made slaves out of captured enemies, they just hadn’t found a way to write down a bill of sale/trade yet. There doesn’t seem to be a lot of evidence showing people in ancient times being gentle or kind outside their own small group/tribe when it came to trade goods/wealth/property, etc...