If git auto-added them from the get go, you would have used a pattern in your personal or project gitignore to skip them, or you would have put them into an ignored tmpdir, or you would keep them on a separate branch if you intend to make them part of the repo. None of these steps would have felt painful or out of place. You see files you don’t intend to include in `jj status` and you’d do whatever is appropriate to exclude them.
I’m with Steve here. I too was skeptical, and it’s turned out to be a complete non-issue.
> and I'd have to something akin to git's filter-branch to clean them out again to avoid that bloat
It’s even more trivial than you might imagine. If you have a commit that added files you don’t want to include you `jj edit` that commit, remove them however is appropriate (rm, mv, add to gitignore, whatever), and… there is no step 3. Later commits are instantaneously and automatically rebased so as to not include those files. Tools like filter-branch are completely unnecessary.