When we say that what is impossible to man is not impossible to God, we are talking about extreme cases.
That means to say that an impossible case to man would mean that it is illogical, irrational and unimaginable.
One example would be the conception of Jesus in the womb of Mary by the power of the Holy Spirit without human intervention.
That is impossible to man but not impossible with God.
But in the 1st reading, what was possible became almost like an overwhelming task.
Though the rebuilding of the Temple was not without challenges and difficulties, the people were losing the spirit and the will to keep the work going.
They were losing perseverance such that they were starting to think that the rebuilding of the Temple was going to be futile.
Hence the Lord God had to command the people: To work! I am with you and my spirit remains among you. Do not be afraid!
So even though nothing is impossible with God, still He wants us to put in our effort to accomplish what is humanly possible.
Without God's blessings, what is possible will even seem like impossible.
But with God's blessings, we will not labour in vain, and anything will be possible.