金光明最勝王經疏 3.29

3.29

贊曰：正辨常住。如如是諸法之真性，云是根本。無形相故如虛空，所以名常。是故說常總結。

The commentary says: This directly explains permanence. Tathatā is the true nature of all phenomena, hence it says "it is the root." Because it lacks physical form, it is like empty space, which is why it is called permanent. "Therefore said to be permanent" summarizes and concludes.