It is similar to how I thought, biofilms make it easier for bacteria to survive and even phages are not always successfully in defeating them. The item does say that phages can clear some biofilms or partially clear them however without total clearance and destruction of the persister cells the infection will not be eliminated. I know this from experience because I did clear the biofilm, it made me so ill and the persister cells restored the biofilm some hours later. Persister cells embed themselves in the digestive wall and are difficult to eliminate. Phages in the blood might not be able to access them, especially if the immune system cannot get to them.