I think I know what is confusing you, so I'll try & explain.
Motor cars use the chassis ground as the circuit return . This halves their wiring.
Originally,they used positive ground. But this caused problems with electrolysis (rust). So modern cars use negative ground.
That charger is ex telephone exchange (very nice, by the way (can I have one??)). Telephone exchanges run positive ground.
Us humans are used to considering ground as being negative, simply from our motor cars.
It really doesn't matter. As long as the positive lead from the charger goes to the positive battery terminal, and negative to negative, it should just work. Just be aware that the metal case of the battery charger is connected to the positive battery terminal. This can probably be fixed, by the way. There's probably a heavy lead bolted to the inside of the case and connected to postive which can be removed. Otherwise, don't sit your inverter on top of the charger. The inverter probably has negative connected to the case.
Clear as mud, I know.