Sugar won't dissolve in cacao butter
Sugar won't dissolve in cocoa butter.  Or in coconut oil, for that matter.

When making chocolate, the sugar is smoothed and kept in suspension by prolonged grinding, conching, which is really a mechanical process... and one of the reasons making actual chocolate at home is very rare, absent specialized equipment, as the sugar crystals will not dissolve and this leaves a gritty product.

You might have better luck with powdered sugar, since it's already more finely ground.  Or superfine or castor sugar, perhaps.  I see that the recipe calls for coconut sugar, and I'm not sure there's commercial powdered or superfine, but you might be able to grind it more finely for a better result anyway.  For powdered or castor sugar the amounts in volume will be a bit different - as the sugar will physically settle differently, plus the addition of cornstarch for powdered - but weight should be the same and there are conversions for volume measurements.

It may also help to cool the cocoa butter/coconut oil mix down to a thicker consistency (semisolid, maybe like softened butter, something of the sort) before mixing in the sugar, as the thicker texture should help keep the sugar in suspension rather than letting it settle out before the mix finishes cooling.  With the cocoa butter/coconut oil mixture in that thicker state, the incorporation would look more like creaming.