Did you read parent comment? How is dealing with the thousand-line change not possible with what they described (hint, it's totally possible, no index needed)?
Well, that's the weasel word. In my grandparent comment, I proposed the "alike", didn't I?
Nowhere did I say, just remove the index from Git, but don't replace its functionality with any other representation or mechanism.
In git, we can do that today in such a way that the index is only temporarily involved:
$ git commit --patch
... interactively pick out the change you want ...
$ # now you have a commit with just that change
It is not some Law of Computer Science that the above scenario requires something called an "index", which is a big archive holding all of the files in the repo, where these changes are first "staged" before migrating into a commit.
