The reference is to the question in chapter 13, when the disciples couldn't figure out who was going to betray Jesus. Simon Peter knew to ask *that guy* to find out - the one whom Jesus loved, the one leaning on his breast.
Simon Peter, Can't you hear my heartbeat? (sings Peter Noone)
No wonder John ended up a "hermit" on Patmos for the word of God. With his insistance on accuracy in reporting, how popular could he have been at the church socials and prophecy conferences. John -- the one who was given the Revelation (discovery) of Jesus Christ for real. Gee, I can't imagine why. /s
John 21
20 Then Peter, turning about, seeth the disciple whom Jesus loved following; which also leaned on his breast at supper, and said, Lord, which is he that betrayeth thee?
21 Peter seeing him saith to Jesus, Lord, and what shall this man do?
22 Jesus saith unto him, If I will that he tarry till I come, what is that to thee? follow thou me.
23 Then went this saying abroad among the brethren, that that disciple should not die: yet Jesus said not unto him, He shall not die; but, If I will that he tarry till I come, what is that to thee?
24 This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.
25 And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen.
"Lord, which is he that betrayeth thee?" It's these folks who ran off with their sloppy, fake but spiritually exciting, amplified human interpretation:
Then went this saying abroad among the brethren, that that disciple should not die...
Ironically, it was a blessing given to the one who stands his ground by insisting on the plain words.
Luke 20
17 And he beheld them, and said, What is this then that is written, The stone which the builders rejected, the same is become the head of the corner?
18 Whosoever shall fall upon that stone shall be broken; but on whomsoever it shall fall, it will grind him to powder.
noun: muller
a stone or other heavy weight used for grinding artists' pigments or other material on a slab.