No no, it is infact the case that Karlach has a literal engine for a heart.
Zariel used her as an experiment to see if replacing a heart with one of her infernal engines would work. Which it did. It enhanced her power greatly and could be supercharged with Soul Coins to produce even more power.
Keep in mind that it's not just a simple engine, but one that runs off of magical energies and fantasy materials (Which is partially why there's this need to service it more frequently when outside the plane of Avernus leading to Karlach's story arc)
It might seem a little ridiculous, but that just reflects upon Zariel herself. She doesn't think of people as people, but as mere tools. So why not replace a part in a tool for a better one that would make it more effective?
Honestly, it's more of a surprise that this sort of thing isn't more common. Since Zariel isn't the only one developing technologically advanced machinery, there's constructs like Bernard and the Steel Watch that exist, the idea of replacing limbs/organs with more effective machinery is not a particularly wild idea (We see this in real life with prosthetic limbs while games such as Cyberpunk 2077 take such a concept further) so it's strange that only Zariel seems to be experimenting with it with her infernal engines.
