Someone may correct me if I'm wrong, but I believe the passage in question is referring to those who have received the Holy Spirit and then still turned back to sin.
The impossibility of re-entering repentance is referring to specifically this life. And how could they, after walking in Truth, repenting, and having Christ dwell within, turn back to repentance?
They have crucified Christ all over again, denying His Power of Resurrection by their actions. Again, I might be wrong, but I believe that may be the very essence of blaspheming the Holy Spirit.
But the hope of Christ is, that after much hell to pay for rejecting Him after true intimacy with Him (imagine a wife cheating on her husband, a sin punishable by death in the Old Testament), they will go on to receive salvation in the next life.
Millions if not billions die without repentance. Their hope is not in this world, but the next, where the Lord will reconcile all things to Him.
One can only speculate on the next life unless given revelation from God, but I imagine it is quite similar to this one, albeit spiritual and not physical.
Thus, scenarios in which people receive correction, learn, are edified, and grow in the knowledge of Christ that leads to repentance are all likely. He's confirmed to me several times, this life is not the end.
But who can know these things aside from the Lord? Let us focus on Him today while we still live and breathe, and let Him deal with those who have fallen asleep.
Read:
Diabolical Doctrine: Salvation Is Possible Only in This Life for more on this subject.