(yes, there's a division that do organic stuff instead of replacing your pancreas with a radiator).
Isn't that heretek?
Not necessarily. It's all about corrupting the Emperor's design. Do you improve kidney function by crafting new enzymes or improving the ones already present? Not heretek. Do you improve kidney function by making an entirely new kidney that doesn't even really look like a kidney or function using the same processes as a human kidney? Heretek.
The Emperor had the skills and license to create "new human organs" like what the Space Marines have. After him, that level of tampering is both incredibly difficult and viewed as heretical. (In the current Imperium, the human form is thought of as sacred and Emperor approved: humans are exactly as the Emperor wanted us to be. Fooling with that beyond juvenant treatments and some chemical shunts is considered heresy.)
There's also the stricture in the Mechanicus that is old is sacred, what is new and created is heresy. Found an STC for improved biological human lungs? Sacred! Invented new biological human lungs in a test tube? Heretek!
That said, it's only Heretek if you can prove it, and that can be hard to do with internal organs.
Wait wait wait, 40K does have cloning, at least I think so, I recall reading something about when not enough criminals where at hand to lobotomize and turn into servitors clones would be used instead.
As for cell cultives, the Mechanicus must have this for growing all kind of replacement organs (yes, there's a division that do organic stuff instead of replacing your pancreas with a radiator).
Most if not all tissue the Biologis branch of the Mechanicus grow is genetically engineered. They use vat grown gene-brutes to make praetorian battle servitors, lifter servitors and similar large servitors. They use Ogryn when available though, so there's probably a difference in quality between vat grown and womb born bodies for servitor purposes.
I do recall one book (Siege of Castellax, good book, I recommend it) mentioned that Embryo Farms where a thing, so the Imperium/Chaos certainly have access to artificial womb tech, which is presumably the same as used in vat grown body production, so it's possible that vat grown doesn't actually mean cloned anyway.
I think that's really the difference. Growing stuff in a vat != cloning in the mind of the Imperium, because those things weren't human anyways. As I said, I think it's all about the corruption/alteration of the existing human form. No one cares if you grow a non-human in a vat to walk your luggage around. They DO care if you cause your valet, who is human, to grow a whole bunch of extra organs. (Extra organs being different than say, increasing their muscle mass or fusing their rib cage.)