Would you consider a space marine in his armor to essentially be a cyborg?

Only if he is a Techmarine. Standard SM implants are bionics, not cybernetics.

I'd agree with Jargal, cyborgs have by definition replace biological parts with cybernetics. Although their connected to their armor, they're not part of it and it hasn't replaced part of them.

Generally speaking though, almost all artwork of 40k depicts characters with some degree of "cybernetic" something. Be it tubes in the head, metallic bits on skin, augmetic eye replacements, limbs, etc.

Obviously in terms of DW, there are many of those things as actual gameplay "things" a player can have (e.g. the benefits of augmetic limbs). But arguably, it should be considered appropriate for a player to say their character has some of those cybernetic stuff that is so common in the setting, and it just provides no net bonuses/negatives, with nothing actually interacting with them. So in my mind, any 40k character could essentially claim to be a cyborg if the player/GM deigns it to be so.

What is the issue driving this question? Is it just some terminology confusion, some question on how a character in a setting would react? Some rules mechanic saying cyborgs suffer more?

Still, wouldn't the interface of the Black Carapace with the Power Armour be considered a sort of "cybernetic" relationship. The Carapace does interact with the suit, doesn't it?

Just curious.