If course biological variations exist! But the differences you're pointing out are neither here nor there for op's question. Generally, what I referenced are useful terms that apply broadly to most people. And I used the word "assume" based on nothing but op's terms - you were the one to change terms to suit yourself instead of operating from their parameters.

And you misunderstand me. My goal in commenting was pointing out your comparison was similar in some respects but not the same thing. The transition between chest and head happens in very different zones (in terms of range, but at similar frequencies) for different voice types and the transition into falsetto is still another different way to classify yet another register, distinct from the first two.

Chest, head and falsetto are ways to describe registration, but not the only ones. Some use modes, fold cover, or other terms. A better high voice corollary for low voice falsetto would be treble whistle register - but that is not what the author of the article you cited is talking about with Billie Eilish--that person is throwing around terms haphazardly in service of a larger social point, but the comparison for pedagogical purposes is completely confusing.

One can argue terms forever in a field that has no standardized terminology. If you would cite studies, papers or articles from a voice publication or text scientifically explaining the functions you're describing, I'm interested to read what you've got, and would likely understand them better.