Steel Defenders are "biological" Constructs?

[JackPhoenix]

The "doesn't require air, food, drink, or sleep" from constructed nature isn't part of the stat block itself, but it the fluff bit describing the monster in question. Being a class ability instead of a proper monster, Steel Defender has only the stat block and lacks the fluff text. The same is true of the various spirit summons from TCoE, or artificer's Homunculus Servant. Iron Defender (which is essentially the same thing, but as a monster) DOES have constructed nature in its description.

It's most likely a quirk of how the text is structured. A bug, not an intended feature.