That certainly accounts for part but consider that Paul also said that "we who are alive and remain until the coming of the Lord will by no means precede those who are asleep." Meaning that those who sleep included even those before the gospel went out, who began "to be absent from the body and to be present with the Lord" even since Adam, as those are the terms.
So then, yes, Paul's word to the Thessalonians would certainly have meant what you said. But with all that he said combined and the rest of the scriptures, all are included even of all times.
Paul is saying those who are still physically alive when the Lord comes again will not be physically resurrected and bodily changed (made immortal & incorruptible) before those who have already physically died in Christ. Those who have died (sleep) in Christ, before His cross and resurrection are spirits as living souls that return with Christ. Those of faith who died before the first advent of Christ ascended to heaven with Christ after His resurrection. That's why Paul can say "to be absent from the body and to be present with the Lord." Because Christ has given us assurance that whosoever lives and believes in Him shall never die. And even Abraham, Isaac and Jacob are counted with those alive in heaven, being of the resurrection through Christ, they are as the angels of God in heaven, spirit beings.
Ephesians 4:8-10 (KJV) Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men. (Now that he ascended, what is it but that he also descended first into the lower parts of the earth? He that descended is the same also that ascended up far above all heavens, that he might fill all things.)
1 Thessalonians 4:14 (KJV) For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.
Matthew 22:30-32 (KJV) For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven. But as touching the resurrection of the dead, have ye not read that which was spoken unto you by God, saying, I am the God of Abraham, and the God of Isaac, and the God of Jacob? God is not the God of the dead, but of the living.
In 1Cor 15 Paul takes great pains to show believers are not only physically resurrected and changed when the last trumpet sounds, but we shall also be raised to heaven a spiritual body when our physical body is dead. Proving again "to be absent from the body and to be present with the Lord."
1 Corinthians 15:35 (KJV) But some man will say, How are the dead raised up? and with what body do they come?
1 Corinthians 15:44-50 (KJV) It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body. And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit. Howbeit that was not first which is spiritual, but that which is natural; and afterward that which is spiritual. The first man is of the earth, earthy: the second man is the Lord from heaven. As is the earthy, such are they also that are earthy: and as is the heavenly, such are they also that are heavenly. And as we have borne the image of the earthy, we shall also bear the image of the heavenly. Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.
So yes, all are included in Christ both spiritually and physically. Because in life we have part in the first resurrection, in death every believer ascends to heaven a spiritual body. Then when the last trumpet sounds every believer will be physically resurrected with immortal and incorruptible body of flesh, with our eternal spirit to inherit the new earth with Christ forever.