THOR wields the MCU’s most powerful weapon and then destroys it, to understand the given statement lets rewind the story and peep into it
THOR-1

He inaugurated when Galactus transformed the god of thunder into his herald due to the impending Black Winter, a threat that has the potential to destroy the entire Marvel Universe and the defeat of this so-called plaque is only possible by powering up Galactus with five hidden planets whose energies will transform the Devourer of Worlds into something even more powerful.
THOR 2

This starts with Galactus consuming the first of these five planets while the unique energies drove the cosmic giant to consume the planet until he annihilated it. The relationship between Thor and Galactus is one filled with hate but driven by necessity and now the return of Beta Ray Bill makes things much more complicated
inference :

Beta Ray Bill comes into existence believing that Thor has fallen under Galactus’