Adding "truth" to Jesus' answer is to directly answer the questions of the twelve of them. In other words, in the future, the twelve apostles will become representatives of all Israel and have the authority to judge Israel (limited scope-Israel).
As for what you will get after you leave everything behind and follow the Lord, continue to read verse 29. However, the Lord did not promise them "throne and authority."
Just because the Lord doesn't agree doesn't mean there is no "throne". Depends on who the Lord gave it to.