Jesus commissioned the disciples, empowering them to be ambassadors who announce that sins are forgiven to those who believe the gospel, and that sins are not forgiven to those who reject it. It is not about the disciples granting forgiveness on their own, but about proclaiming God’s forgiveness through faith in Jesus Christ. The Scripture you reference emphasizes that ultimate authority to forgive belongs to God, while the disciples are agents tasked with sharing the message of salvation.
The disciples were filled with the Holy Spirit (as you’ll read right before John 20:23) and when we accept Christ, we are filled with the Holy Spirit, which gives us the same power that raised Jesus from the dead