Steel Defenders are "biological" Constructs?

[Cheesegear]

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

Tasha's has been out for quite some time. If it was supposed to be Errata'd, it would have been by now. There are other Constructs and Undead and Oozes that need to do...Stuff. It's different for every creature so I assume it's on purpose. Every creature unless specified needs to eat, drink, sleep and breathe. Even the magical ones.

As has already been pointed out; "Immune to Exhaustion." actually covers three out of the four, without actually saying it.

Perhaps the SD needs something from the artificer's body- like blood, which is how it controlled (and powered?), but also explains why it needs oxygen.

Something I came up with was that the Steel Defender is powered by some sort of furnace. If you douse the thing in water or smother the oxygen in the room with some other gas, the furnace or power source probably wouldn't work anymore.