I generally wouldn't want to get involved in this argument, but above leeyiankun highlights a response about the Heart of the Universe and then proceeds to compare IB and the UN completely ignoring GH's main statement that he himself highlighted.
He specifically asks how it can be proven, so I'll do that now. The person with the HotU has the power of author fiat. That is the point of The One Above All and why it consistently is presented in the form of the creators of Marvel Universe. It is not an in-universe hack. Anything on that level is literally the author saying he wants this to happen, and TOAA, THotU, or the Living Tribunal if neither of those is in play doing it regardless of what is going on in the story.
He specifically asks how it can be proven, so I'll do that now. The person with the HotU has the power of author fiat. That is the point of The One Above All and why it consistently is presented in the form of the creators of Marvel Universe. It is not an in-universe hack. Anything on that level is literally the author saying he wants this to happen, and TOAA, THotU, or the Living Tribunal if neither of those is in play doing it regardless of what is going on in the story.