Steel Defenders are "biological" Constructs?

[JackPhoenix]

It's not part of the stat block because it's part of the Animated Objects traits. Is the Steel Defender an Animated Object? I say it is, thus Constructed Nature figures in, and it doesn't require air, food, drink, or sleep.

No, it isn't. I have no idea how you came to the conclusion it's an Animated Object, it's not called one at any point. You'd have more of a case if you'd called it a homunculus... not that it is called one at any point, either... because of the Iron Defender.

Your apparent contention is that the Steel Defender is not an Animated Object. I'm not sure what else you'd call it - given that the entries under Animated Object don't call themselves anything different... the Animated Armor is 'medium construct', identical to the Steel Defender entry.

A Steel Defender. That's what it is called.

I'm also not sure where your definitive stance of 'which monsters from class features (and spells) lack completely', given that the stat block explicitly states 'medium construct'.

That's size category and creature type. That has nothing to do with anything. A griffin isn't a centaur just because they are both large-sized monstrosities.

Rule it as you wish, but my table is ruling the Steel Defender is a construct - because the stat block states it is, and that it is an animated object because it is an object, not a living creature, and it can move, thus it is animated; and animated objects have the Constructed Nature trait.

It's a construct at every table, that's the creature type. It's not an object, it's a creature, and it's explicitly called a creature multiple times in the description of the subclass ability. It's not an Animated Object, which is a specific sub-category of constructs. I really don't understand the logical leaps that led you to this conclusion.