The context just suggests the author has no idea what they are doing. That's not how any electronics engineer would label schematic and it is clearly wrong way

The context just suggests the author has no idea what they are doing. That's not how any electronics engineer would label schematic and it is clearly wrong way to do it. Typically there is voltage reference selected and marked somewhere on the schematic. By tradition, if you don't mark it, it defaults to negative battery terminal. The rest of voltages would be understood in comparison to the reference point. More complex circuits might need multiple reference points. For example, a PSU converting from 230V AC to 12V DC will need at least two reference points.