Well, actually it's not exactly a PoS coin, its consensus mechanism sometimes is called PoWS (Proof-of-Working-Stake). A user is not just collecting a reward once in a month (open the wallet - stake - close for another month), for example. They use their balance to actively stake making their wallets behave similar to PoW miners (by utilizing the weight in a proper way, that's what the concept of block sizes is all about). Which pushes up the difficulty dozens of times higher. On the other hand, the same approach that makes PoWS work (a flat reward or, in case of HyperStake, the max subsidy) also controls inflation. HyperStake is one of the pioneers of the PoWS world, it's like a technical standard in this field.