Indeed, the power Pei Xuanjing demonstrated was actually different.
In the process of ascending to become a True Immortal, he indeed had touched the domain of the Immortal Path, which was qualitatively different from the mortal domain on par with that of the Supreme Powers, but this did not mean that the Supreme Powers were undefendable against True Immortals who had not yet undergone a transformation.
If merely touching the Immortal Path domain could exert overwhelming pressure on the Supreme Powers, then what would be the point of the Obstruction Tribulation? It would become utterly meaningless.
In fact, that was not the case. The reason was not that the Obstruction Tribulation was meaningless, but that Pei Xuanjing's cultivation process was different from others.