I think it was more targeting the client ISP side, than the VPN provider side. So something like having your ISP monitor your connection (voluntarily or forced to with a warrant/law) and report if your connection activity matches that of someone accessing a certain site that your local government might not like for example. In that scenario they would be able to isolate it to at least individual customer accounts of an ISP, which usually know who you are or where to find you in order to provide service. I may be misunderstanding it though.
Edit: On second reading, it looks like they might just be able to buy that info directly from monitoring companies and get much of what they need to do correlation at various points along a VPN-protected connection's route. The Mullvad post has links to Vice articles describing the data that is being purchased by governments.