I think this is such a heartbreaking scene, because (in my reading) Mokuba's being a bit of an unreliable narrator here. Not entirely, but... mostly? This is what he needs to believe, but it hasn’t been true for a really, really long time. He has to believe it, though, and has to believe this is the “real” Seto in some way, because otherwise, he is actually totally, completely alone (and has been for years). 

The reason Mokuba is the final piece to Seto “putting his heart back together” after the shadow penalty coma is because he’s the one holding onto this dream, he’s the one who still believes in this piece of Seto, is actually the only person alive who’s known Seto long enough to remember this and believe Seto can find himself again. 

(Also reading this as Mokuba being a bit of an unreliable narrator lessens the, you know, retcon whiplash you get from Kaiba being changed from a one-off villain to a sympathetic recurring antagonist and eventual anti-hero.)