I suspect that most of the first readers of John's book thought they were in "great tribulation" then. Nero (or someone like him) had tried to kill John by boiling him in oil - but failed. So he banned John to the island of Patmos. (Little good that did!)
Question: do the two words together: "great" and "tribulation" always have to mean the days Jesus spoke of after the abomination? It seems you think so. Remember, those two words were NOT ENOUGH for Jesus: He went on to say that there would never be other days such as those: not in history or ever in the future. Since this is true, that leaves the two words, "great tribulation" to refer to any time of severe persecution, such as when Nebuchadnezzar forced people to bow to His image upon threat of death. "Tribulation" means pressure - pressure to do something wrong in the eyes of God, such as to take a mark when you know that would doom you to eternal death. "Great" means just as it says, from the Greek "mega" which we get magabyles - meaning a whole lot - or megawatts - again meaning lots of.
What am I saying? Seeing "great tribulation" in scripture does not always mean the days Jesus spoke of that would come after the abomination that will divide the week. Did you not notice this verse?
Rev 2:22 Behold, I will cast her into a bed, and them that commit adultery with her into great tribulation, except they repent of their deeds.
This was a message to a certain person and to those who were sleeping with her. Was God planning on keeping her alive for 2000 years - or perhaps only that God can create "great tribulation" any time He wants?
Next, if you study the bible, you will find that Jesus did not put a title on the last half of the week, only saying "those days" of "great tribulation." People say "the great tribulation," but that is not scriptural.
Do you have any idea how many this group is? They are billions of people! Perhaps 50 generations of believers, plus the final generation of believers plus all the children alive at the time: billions. John is not going to be close enough to recognize any single person. I will give him leeway to NOT know who they were. Perhaps he guessed but was not sure.
I know they cannot be 70th week people for several reasons: first, they are not souls, they appear as normal and complete people. If they were martyrs, we could be sure John would have said so. Next, the rapture crowed will be MANY MANY TIMES BIGGER than the martyr group. Next, the martyrs of the days of GT do not even begin to show up in heaven until chapter 15! Next, every descriptive term John used to describe them SOUNDS exactly like the church.
Next, John wrote in chapter 1, "I John in THE TRIBULATION..." showing that he was in "the tribulation" in his day. But John saw the group long after his day in the vision, a time still future to us. We can be sure, "the tribulation" is still ongoing and martyrs are still be added. Since the Holy Spirit through John has already told us that God considers the church age as "the tribulation," is it too far a stretch that since there will be around 2000 years that He could call the church age as "maga tribulation?"
I had a very similar problem: first, God TOLD me this group was the raptured church, so I had to believe it. But I was greatly troubled by the words. I pondered this for at least a year, trying to come to grips with "great tribulation." I finally did. Those two words together don't have to mean the days Jesus spoke of - and in this case, before the 70th week begins, it does not and indeed cannot.
The "ELDER' who is a representive of the Raptured church now in heaven knows that this bunch of people is not the church Since they ARE the church, this is a false statement. He told John EXACTLY who they were: they came out (one by one) from the church age, "the tribulation" according to John, and here Jesus added mega tribulation, meaning year after year after year meaning LOTS of tribulation.
Finally, since I know by Paul's writing that the rapture will be the trigger for the Day of the Lord, and that the 70th week then starts days after the DAY starts, and that the days of GT are not until years later in the last half, I know with certainty that these are NOT 70th week martyrs.
It is good you question this: I spent years questioning it. But I bugged God about it almost constantly!