Yes, that’s idiomatic in JJ. Honestly, I’ve been recommending the same workflow (first commit, then make changes) with Git too since years

https://arialdomartini.github.io/pre-emptive-commit-comments

If you want to have a workflow similar to Git with index, check out the Squash Workflow: basically, you would edit your files in a disposable commit having the same purpose of Git’s index.