Qin Mu could execute Heaven and Earth Mudra not because of the overlord body, but because of one sentence that First Ancestor had said, and that sentence was 'You and I are the same, orphans of Qin family.' Hearing these words made him feel the same and his frame of mind resonated with First Ancestor.
He could experience the destruction of Founding Emperor Era, and as the only orphan of Qin family in this world, he could feel the helplessness as he carried up this heaven and earth by himself.
This kind of feeling was how First Ancestor felt, and that was also how he felt.
Not only that, First Ancestor wanted to carry Founding Emperor Era and not let the era be wiped out. What Qin Mu wanted to carry was the world today, the era of Eternal Peace.