I think you are taking the poster too literally when “dead people” is used. I read it as “people that have died” and in the context of the discussion that would make them the “saints in Heaven”.
Anyway, that is how I took it.
No one is in heaven, yet. From our point of view. All those who died or are alive at the second coming will be transformed or resurrected at the same instant. The event takes place from the perspective of the believer at their death, but from the perspective of the living, all at the same trumpet sound in the end. Both are true statements, because the very thing we call “time” is just an illusion of a static singularity to beings outside of time. Everything that is, is all past tense from the point of view of God, but present and future for limited man.
Thus, the instant someone dies, they are outside time and space and present with the Lord at the end of time. Thus, they cannot pray for events which are already over and ended to which they were unaware when in progress and for which, because it is now the end of time for all the believers, including us.
This is why we are said to be sleeping, from the perspective of the living. And present with the Lord when dead. Both are true. But both prevent the sleeping from being aware of the living or even concerned with our events.
There is one exception. Hades. The dead who are not in Christ are aware of the activities of the living. There is no rest for the wicked.
See: 2 Samuel 7:12, (and likewise 1 Kings 2:10, 11:43, 14:20, 31, 15:8, 24, 16:6, 28, 22:40, 50 etc. etc.), Job 10:21, 13:12-15, 14:21, Psalm 6:5, 13:3, 30:9, 31:17, 49:17-20, 88:10-11, 115:17, Ecclesiastes 9:4-6, 10, Isaiah 38:18, John 3:13, 11:11-13, Acts 2:29,34, 13:36, 1 Corinthians 15:51.