They were probably referring to the fact that the 489 cases use crush sleeves for bearing preload and that when disassembled, you're "supposed" to install a new crush sleeve. Other than that caveat, they are completely rebuildable.
He might have been referring to the cone-type sure grip, which are not easily rebuildable compared to the clutch-type. The cone sure grip requires machining to be rebuilt, where you can just throw new clutches into the other.
That's kind of what I thought. I had a 742 in the 8.75 I had in my old charger, I had it rebuilt(not by me) but I couldn't fathom a single reason a 489 couldn't be rebuilt.