However, this assumes that you transfer the contents of your attic between platforms. That's not usually recommended; if you want to transfer between machines you can use the regular 'create patch' feature (Repository > Create Patch) or use a shared fork.
Usually, SourceTree avoids user settings like this because they can cause unpredictable behaviour. Since the attic is really supposed to be local to a machine, I do not wish to make the behaviour less predictable just to support this unusual edge case, especially since there's another way to go about it.
hgattic is basically a different implementation than Mercurial Queue but serve a similar purpose: save local changes/patches somewhere and I can easily apply or unapply.
Mercurial Queue will respect this mercurial use setting and TortoiseHg shelf function will also respect this setting. That's why I think hgattic should respect this one as well.
Since it is just a user setting, user can turn on/off either in their user configuration or in their repository configuration, I don't think it will cause unpredictable behaviour.
On the contrary, current behavior is not consistent, since I turn on this setting, but it doesn't take effect, just make me think it's a SourceTree bug