I think any confusion is a matter of definition, which is pointed out in the first few lines on that site.
It's one of the basic things of electronics that electrons can only 'travel'/move when there are two terminals. If you got a (+) then you have to have a (-) somewhere. That 'somewhere' ain't always where you think it is, but it's always there. Otherwise, those electrons ain't moving. Those two terminals, (+) and (-) are easy to see if something is symmetrical, has two sides. When there seems to only be one side, then you just aren't seeing the other one, look some more.
- 'Doc