You're assuming these under the altar lived and died after the first advent of Christ. If that were true, why are they seen under the altar and not in heaven as John says of those who were martyred for the witness of Jesus (Rev 20:4)?
Revelation 6:9 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
10 And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?
11 And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
I take this passage to be involving two locations, heaven and the earth. I reason it like such.
I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held---something that happened upon the earth which led to their deaths--and now they are in heaven, not sitting on thrones reigning with Christ, but under the altar.
And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?--- They are in heaven when they are crying out like this.
And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season---they are still in heaven and still not depicted as sitting on thrones reigning with Christ, then are told they need to rest a little season before their blood can be avenged,, in order to fulfill, their fellowservants also and their brethren, that should be killed as they were. Then after this little season is finished, God will avenge them all, including their brethren that are killed during this little season.
It is then a question of, what happens every time one of their brethren back on earth is killed during this little season? When they enter heaven in a disembodied state, do they join these under the altar that are told to rest until this little season expires, or do they instead get to sit on thrones and reign with Christ until this little season passes? You then end up with God not being fair. One group of martyrs have to rest until the little season expires, the other group gets to sit on thrones and reign with Christ. Keeping in mind, that resting, in no way shape or form is depicting anyone sitting on thrones reigning with Christ.
It doesn't matter who these martyrs are meaning that are under the altar. The point is, they are told to rest until this little season passes on earth, not sit on thrones and reign instead. So why would their brethren back on earth who are killed during this little season not join these resting until this little season passes once they enter heaven in a disembodied state, but instead get to sit on thrones reigning with Christ?
The main point being, Revelation 20:4 is not meaning during the era of time involving the fifth seal, the 5th seal also involving this little season, it it meaning after this era of time involving the 5th seal. It would basically look like this. The 5th seal followed by the 6th seal, then followed by the 7th trumpet and the last 7 vials of wrath, followed by the 2nd coming, followed by Revelation 20:4. Thus Premil not Amil.