Circuit with dual power supply
Looking into the datasheets, I found out the comparator (LMC6772) is able to drive the SCR (S6S2) by itself.


You are mistaken about that. The LMC6772 has open-drain outputs, which means that they can only sink current, not source it. However, the SCR requires current to be driven into its gate terminal for triggering. Therefore, the PNP is required to convert a current sink into a current source.

It really doesn't matter to the transistor whether the current is returned to ground or to the negative rail. Either way, the loop is completed through one or both power supplies.