When I read this verse I assume it’s based on JTB being alive on earth. Only what is perfect is in Heaven.
”Nothing impure will ever enter it, nor will anyone who does what is shameful or deceitful, but only those whose names are written in the Lamb’s book of life.” - Revelation 21:27
So while JTB is the greatest human to walk the planet outside of Jesus, while on earth, he is still not perfect and to some level he was sinful and imperfect. After all the prophet Isaiah writes:
”A voice cries out: "In the wilderness prepare the way of the Lord, make straight in the desert a highway for our God.” - Isaiah 40:3
My guess is Jesus is noting that while on earth since JTB isn’t perfect he is lower than the lowest in Heaven. But once he is perfect in Heaven, he should be right up there as one of the greatest ever