The "Optimize Mac Storage" functionality you're referring to is not enabled by default.
You can manage settings for this and other storage optimization features in About this Mac -> Storage -> Manage, and System Preferences -> iCloud -> Option... (beside iCloud Drive).
Apple does not make this clear on their Sierra micro-site, and acts as if it either enables by default, or asks you with their typical confusing dialogs to enable it when low on space (which most users would get confused and click enable).
