I mean that they’re not necessarily part of the --message parameter to `git commit`, but instead part of the --trailer parameter. I don’t know how VSCode is programmed, but it seems plausible that trailers are handled separately from the message parameter.
We're talking about Git here. The question is not "how VSCode is programmed", the question is "does Git have a special field for commit trailers". The answer is no. Git stores the trailer as part of the commit message.
If you look at the comment I’m responding to, it is in fact about how VSCode is programmed; specifically, a possible reason why the Co-Authored-By trailer doesn’t show up in VSCode’s commit message box.
https://git-scm.com/docs/git-commit