So the Landraider is its own unit, that is what you said and the same thing the codex said as unit type and unit composition. So here is what the power says "Movement phase. The Librarian, and any unit he is with, are
removed from the tabletop and immediately placed back
together anywhere within 24" using the deep strike rules. If
the Librarian travels alone, there is no risk, but if he takes a
unit with him, there is a chance something will go wrong. If
the deep strike attempt scatters and a double is rolled, one
member of the unit, chosen by the controlling player, is
claimed by the Warp and removed as a casualty (the
survivors scatter normally)."
So any unit he is with not joined.
I know its cheesey and such but that is how I read it.
