Even if was not directly stated, Arishem was before anything, the universe, and even before spacetime itself(since the infinity stones dont exist then) so the celestials must've made everything, including spacetime itself, and since spacetime isnt what is created gradually then it is only reasonable that Arishem first sun feat made spacetime itself.
Arguiung that is just not right. Wong stated the stones formed immediately after the big bang, and from his holographic description energy and matter was formed simultaneously, but none of this existed. So even if Arishem didnt create the bigbang, then all of spacetime, matter, energy, the stones, must've been a collective work of the celestials and that is far beyond 4c.