It only creates a shortage in a world where you have the x-shaped supply and demand curve from econ 101, which doesn’t apply here because the shortage already exists because of zoning/planning laws limiting the construction of new housing. If there wasn’t an existing shortage then no-one would be calling for rent controls. Rent controls just change who the winners and losers are as a result of a given shortage, in Stockholm’s case, existing residents are the winners and new residents are the losers. Existing residents who benefit get to vote to maintain the status quo in municipal elections whereas the people that can’t move there because of rent controls don’t, hence the system is self-sustaining.