part of the "problem" is that rent contracts are unlimited, so if rents are to low, then it may make sense to wait until they rise before renting out, especially in sight of a law that doesn't allow any raises for the time being. but as the article points out, many instead opted to sell outright.
So, to your point, this law of capping rents would then incentivize landlords to keep rental units vacant? Wow, sounds like exactly the kind of effect one can expect from laws suggested by Red-Red-Green in Berlin that lack any common sense or basic understanding of markets.