True to an extent--meaning mostly true in practice.
You can mutate the Git data if you chose to using commands like "filter-branch". "filter-branch" isn't used frequently, since it causes issues with every up/down-stream replica if the data has been pushed/pulled, but it is possible. But, even some commonly used commands like "amend", "rebase", and "squash" cause limited data mutations which are broadly considered appropriate and useful.
You can mutate the Git data if you chose to using commands like "filter-branch". "filter-branch" isn't used frequently, since it causes issues with every up/down-stream replica if the data has been pushed/pulled, but it is possible. But, even some commonly used commands like "amend", "rebase", and "squash" cause limited data mutations which are broadly considered appropriate and useful.