We most absolutely do not deny that the tribulation believers will belong to Christ. All of the Old Testament saints belong to Christ. But the Lord Himself said, "Assuredly, I say to you, among those born of women there has not risen one greater than John the Baptist; but he who is least in the kingdom of heaven is greater than he." (Matthew 11:11)
The fact that there was not one "born of women" that was greater that John the Baptist means that John was at least equal to any and every Old Testament believer, including even Abraham, Moses, and David. But the least of those in "the kingdom of heaven" is greater than John, and this greater than any Old Testament Saint.
This is not a matter of opinion. It was clearly stated by our Lord himself. But it clearly shows that even the least of the New Testament believers is greater that the greatest of the Old Testament believers. This clearly shows that "the church" is something different from just "those who believe."
We see this again in John's statement about the bride in John 3:29, "He who has the bride is the bridegroom; but the friend of the bridegroom, who stands and hears him, rejoices greatly because of the bridegroom's voice. Therefore this joy of mine is fulfilled." Here, John was clearly saying that he was "a friend of the bridegroom, rather that a part of "the bride."
So saying that these tribulation believers are not part of the church is not saying they do not belong to Christ, it is just seeing that they are in the same class as john the Baptist, or the millennial believers. This is a thoroughly concept.