Jesus clearly stated to those around him at the time, that some of those standing there with him, would not see death before he returned. This is unequivocally clear. If you believe that he is yet to return, and he doesn’t, that makes him a false prophet.
Jesus said that some standing there with Him would not taste death before they saw the Son of Man coming in His Kingdom (Matt. 16:28). He was talking about His transfiguration, that took place about a week after He spoke those words (Matt. 17:1-8). Some of those who advocate the Preterist view of prophecy, claim that His transfiguration was the 2nd Coming. But He hadn’t been crucified and resurrected yet, and His appearance on the Mt. of Transfiguration took place before these events happened, so we know He couldn’t have been talking about the actual 2nd Coming. His death and resurrection were necessary to make the 2nd Coming possible.
Also, He said the 2nd Coming would be like lightning, visible from the East to the West (Matt. 24:27), and that all the nations will mourn as they see Him coming on the clouds of the sky with power and great glory (Matt. 24:30). Only 3 men from Israel witnessed His Transfiguration. So His appearance there is not consistent with His own description of the 2nd Coming.
But the 3 disciples saw Him as He’ll look when He returns, and they also saw Moses and Elijah talking with Him, and they heard the voice of God speaking from a bright cloud. To me, this means Peter, James, and John were empowered to step out of time temporarily and witness a conversation at least 2 thousand years in their future between the Lord and two long departed luminaries from the Old Testament with the Divine Presence of God surrounding them. It was not the 2nd Coming, but it was a glimpse of the future Kingdom.
That said, I agree with you that if He doesn’t return, as hundreds of promises in the Bible say He will, that will make Him a false prophet.