I thought it was a separate thing that capitalists could use, but not necessarily capitalist production.
Capitalism is a form of the relation between the society with the means of production and slavery is form of social relation within society. Hence they are neither the same nor mutually exclusive. As history shows us, slavery is perfectly compatible with capitalism. Capitalism refers to the ownership of means of production by private individuals. In slave societies, the slaves were considered property that could be bought, sold and traded permanently on the market and hence slaves were considered capital too. It was common for the net worth of slave owners included the worth of the slaves they owned.
They have similarities, but it’s a distinct mode of production from capitalism. The main differences are here : https://www.youtube.com/watch?v=UoSe66oOKsw