The answer, given at the aforementioned site is speculatory and doesn't mesh.
Peter and John and James went into a mountain to pray with Jesus where they experienced a vision. In that vision they saw Jesus talking to Moses and Elijah. This is what Jesus referred to as the kingdom of God. Both Moses and Elijah were taken to Heaven. We see the proof in the following text.
Where do we read anything about a kingdom? Nowhere. The apostles simply saw two beings who had been taken up. They did not all ascend into heaven; rather, the prophets descended.
Daniel 2 states that the Kingdom of God would be created in the days of the independent nations, part iron and part clay. This followed the Roman Empire and is the kingdom referenced by Jesus here and in the Lord's prayer, "Thy kingdom come...." This tells us that the kingdom wasn't then in existence, but that it would come in a future day.
John and possibly others were promised that they would tarry, or remain, until that day. It's one reason the Romans weren't able to kill John and finally banished him to Patmos. After writing the Apocalypse, he vanished and was not heard from since.
Jesus foretold the destruction of Jerusalem, the scattering of the Jews, the great falling away, the gathering of Israel and the creation of His Kingdom, by which the gospel would be preached to the nations before the great and dreadful day of the Lord. Clearly, all this could not have taken place all within the lifetime of most of the people. Had we a more complete record, we would know if others received the same promise as John.