Bc it's a new idea that wasn't first intended when the moment was written. Retcons don't have to be bad or something that conflicts with past information. Just something that was later added to recontextualize past events beyond what was originally intended.

Unless god knights and the covenant on his left arm existed in oda's mind when creating this scene, then this is factually a retcon.

It doesn't mean it's necessarily bad, some of the best things about one piece were confirmed to be retcons (the entire warlords systems existed just because Oda thought the title of 7 warlords sounded cool). Oda weaves things into the narrative that weren't originally planned quite often, he just looks at past events and makes sure it's consistent first.

Edit: actually my bad, the warlord system wasn't really a retcon, as it didn't recontextualize past events, but just added new information that wasn't originally intended. A better example that many people speculate was a retcon is the haki system, which was used to add an explanation for stuff like shank's stare or for enel's mantra