I suppose it is obvious that if "the dead know nothing" (unconscious nonexistence)
then He could not make proclamation to them. But He did in fact make proclamation to them.
And more than that, He "took captivity captive", that is, led them out of the realm of the dead.
It might be worthwhile for you to do a search in the NIV for the term "realm of the dead".
Some of those references make it very clear that the "dead" are able to hear and speak.
Even the story of the Rich Man and Lazarus illustrates such.
Luke 20:37-39 NIV
But in the account of the burning bush, even Moses showed that the dead rise,
for he calls the Lord ‘the God of Abraham, and the God of Isaac, and the God of Jacob.’[
a]
38
He is not the God of the dead,
but of the living,
for to him all are alive.”
39 Some of the teachers of the law responded, “Well said, teacher!”