Insider Build v0.15.3.
The release notes say: ‘You can now rearrange the order of tabs in the sidebar.’
This allows the sort order to be ‘custom’ set, by pressing on the ‘Drag to rearrange’ icon.
This then allows the tabs (folders and notes) to be manually sorted, by dragging-and-dropping.
However, the moment you then interact with the folders or notes, the alphabetical order is reimposed.