I found this forum thread where the topic is exactly about my question, so it's good to see some conversation about this:
https://forums.freebsd.org/threads/question-about-zil-slog.8...
> (part of the reason is) to group tiny sync IOs together in a persistent form
Yes, that seems to match precisely with what the latest messages seem to clarify in that discussion.
(Just dropping my suggestion for the article author) Seeing that this looks like a reasonable doubt that ZFS newbies might have, it could be an interesting point to mention. The article goes in great detail into the internals, so it probably is already well posed to include a mention about why the ZIL makes sense even when no SLOG is in use.
IIRC above a certain size, you skip the double write; part of the reason to do the ZIL batching is, I believe, to group tiny sync IOs together in a persistent form that can be replayed later, so you can return sooner, rather than having to do lots of random IOs before returning.