I find tech writing skill to be necessary but not sufficient to competent at technical writing. GPT-3 is more than sufficient if my goal is phrases that sound reasonable may or may not capture the essential concepts
Still, it's probably a good idea to have an engineer go over the text after, to make sure there are no glaring errors.
If on, the other hand, you write documentation for Apache Spark, you probably need to be a pretty competent computer scientist, data engineer or data scientist. Maybe a tech writer can go over the text after, but they will probably not be able to understand the content, so can do little beyond formatting, language and structure (and even that needs to be reviewed by a real tech person after).
In the latter case, it may be VERY useful for the tech person to have some formal training in technical writing, in addition to be the kind of engineer that is also pretty good with language and presentation.
It really depends on who the target audience is.
If we are writing for other engineers on the inner workings then yes maybe an engineer from within the team will probably be better suited.
However, if it is a technical file for an external audience or regulatory work I would much rather have an experienced 'Analyst' or 'Systems Eng' (in the requirements sense of the title not sys eng in the US infrastructure sense of the word) do the write up.
We as technical people vastly underestimate how crap we are at explaining things to someone without the very problem-specific domain knowledge.