Read John 21:24 Using Other Translations
This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.
This is the disciple who is bearing witness about these things, and who has written these things, and we know that his testimony is true.
This disciple is the one who testifies to these events and has recorded them here. And we know that his account of these things is accurate.