The verse doesn’t say that these are dead people, it says they “were redeemed from among men, being the firstfruits unto God and to the Lamb”.
These would be the first Jewish people to enter the kingdom of heaven on earth…. This kingdom of heaven.
Luk 17:20 And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God cometh not with observation:
Luk 17:21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you.