No, it is not. All that will prove is that the motherboard ground is connected to the chassis through the PSU. I believe you are confusing "grounding" and "bonding". "Grounded" means there exists an equipotential plane between the chassis and earth. In other words, the potential (voltage) difference between the chassis and earth is 0 Volts. "Bonded" means two conductive parts are connected in such a way as to present 0 Ohms (or as close to 0 as possible) of resistance between them. However, they may not be "grounded" at all.