According to this:
https://blog.bitbucket.org/2012/10/30/building-inline-comments-for-pull-requests-and-commits/
They exist but I couldn't find how to add inline comment within the source code.
Community moderators have prevented the ability to post new answers.
You can only add inline comments to commits, not source. So (if you're a project member) https://bitbucket.org/username/project/commits/12345678 should let add comments by clicking the button that appears on the left hand side when you mouse over a line, but https://bitbucket.org/username/project/src/12345678/file.name has no such feature.
That's because they don't exist.
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