How can compounds be called reduced if nothing is known about the reaction they formed in?
While we don’t know exactly how minerals on meteorites were formed, we can make very educated guesses on the conditions they were formed in. Most notably, outer space only contains very low concentrations of the compound most associated with oxidation on Earth: gaseous oxygen.

But this is not the main reason why the label ‘remarkably reduced’ is accurate here. We will immediately recognise the mineral in question as an ionic compound and given that it is made up of a metal and a nonmetal, we can equally immediately assign oxidation states as shown below.

$$\ce{\overset{+II}{Ca}\overset{-II}{S}}$$

There is no surprise about calcium which generally is found in its $\mathrm{+II}$ oxidation state. The surprise here is that sulphide is the anion which is a rare occurance. On Earth, most mineralic sulphur is oxidised and mainly to sulphate ($\mathrm{+VI}$). This is basically the extreme opposite and caused among others by the high atmospheric oxygen concentration.

Of course, the label ‘remarkably reduced’ can only be applied to some of the elements of the compound. Language-wise, we are looking at the oxidation state and merely stating that a state this low is uncommon.