I guess it depends on your workflow. If you stage each file carefully in git, it's effectively opt-in as you suggest. If you `git add .` though, as many people do out of habit, then it's not going to help much. The same is true really for jj, if you don't edit your commits then yeah, it'll be included by default, but if you craft your commits then you'll spot it.

Also jj doesn't push until you push your changes when backing on to git. This is pluggable though, the backend I work with is not git, and does happen to upload all commits off my machine immediately (although still effectively in a private fork).