It wasn't John the Apostle, son of Zebedee that was being spoken of in these texts. It was "the disciple whom Jesus loved", who Scripture distinguishes as being a different man than John the son of Zebedee.
The only reason why this John 21:24 verse can claim so confidently that "we KNOW that his testimony is true" is that the record of this gospel was produced by a glorified, resurrected person who was incapable of giving a false account by any means whatever. No resurrected person can be mistaken, or tell a lie, or be led astray by any means. They are in a perfected state of righteousness, and can be trusted completely as to their testimony. The same comment is also given in 3 John 12. "Yea, and we also bear record; and ye know that our record is true." Hands down, no doubt whatsoever, "the disciple whom Jesus loved" / aka the beloved Lazarus / aka John Eleazar could be trusted implicitly to tell the truth.
There are no glorified believers on earth, and never were.
Our citizenship is in heaven and that is where is also our glorified bodies and inheritance is found, not on earth, no written testimony is coming from a glorified person on the earth.
Everyone should note and ignore this fellow as this is false teaching, the things he is saying.
Philippians 3
20 For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ, 21 who will transform our lowly body that it may be conformed to His glorious body, according to the working by which He is able even to subdue all things to Himself.
The reason people can confidently know something is true is they have this as a gift from God,
1 John 2:26-28
New King James Version
26 These things I have written to you concerning those who try to [a]deceive you. 27
But the anointing which you have received from Him abides in you, and you do not need that anyone teach you; but as the same anointing teaches you concerning all things, and is true, and is not a lie, and just as it has taught you, you
will abide in Him.
28 And now, little children, abide in Him, that [c]when He appears, we may have confidence and not be ashamed before Him at His coming.
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The fact that some have such wildly divergent views shows they do not have this anointing from God to know the truth.
And are just the blind leading the blind and people who follow them fall into the pit.