So, underneath all the dramatic and flowery language, the argument being made seems to be “if the purpose of our biology is to make its own DNA persist, it wouldn’t make sense for biological chimeras to exist; based on the ability of cells to coordinate even with different DNA, the main goal seems to be human cells cooperating to make a general human form.”

This anthropomorphizing of biological building blocks is ridiculous. Cells and DNA are not in competition over who runs the show because they aren’t sapient. And I fully understand that the scientist making this claim understands that on an intellectual level but I mention it because the backbone of this argument is to conflate the literal and the figurative. The only inconsistency in cells being compatible despite having different “bosses” would be an ideological one and, because there isn’t any actual ideology at play, it doesn’t matter whether it’s consistent when attempting to describe it. You’ve proven a metaphor wasn’t literally true, congratulations.

But setting all that aside, this still doesn’t actually function as a counter argument. If we are to accept the premise of DNA’s authority as literal truth, is this function of unrelated cells to be compatible with each other not a logical extension of the DNA’s will? It more benefits the DNA for the organism to be viable even if that means other DNA also persists. It has a greater chance of reproducing itself if it’s not in a corpse.

Not only does the argument hinge on anthropomorphism, it also hinges on this metaphorical entity being self-destructively spiteful.

Lastly, it is downright comical to mention things like “cells know on their own that the heart goes on the left” when making an argument that a different characterization of biology is wrong based on the existence of rare biological edge cases. Some people’s hearts aren’t where hearts normally go. I’d let this kind of thing slide as a simplification of the truth were this not part of calling out exactly the same degree of simplification from someone else as being invalid.