Yes, it is an old example from before that argument was added.
The only reason for using this filter is that there is something custom you want to do that ACF does not do but in most cases you can replace a pre_save_post filter with an acf/save_post action.
No, the documentation is not all 100% clear in this regard, but this is a user forum and the developers and maintainers of the documentation do not read these forums. If you think the documentation needs clarification then you need to contact the developers.
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