Why is a urea solution hypotonic relative to RBC?
I think you have the conceptual understanding down, but my guess is that the textbook is not representing the situation quite correctly.  Based on the phrasing of the statement, I can imagine they are describing one of two situations:


Imagine a dropping a cell in to a solution of 0.5 M urea with no other solutes.  Urea will diffuse across RBC membranes due to facilitated diffusion.  It won't really factor in to this situation much.  So, the RBC contains proteins and other solutes that cause water to flow in to the cell and the cell lyses.  I don't think this is what the textbook means.
Now, imagine a situation in which you precisely calculated the water potential of a RBC.  You mixed up an isotonic solution of urea and some non-penetrating solutes.  This solution is perfectly isotonic at first, but as the urea diffuses in to the cell, it alters the water potential of the cell and the outer solution leading to osmosis.  I think this is the situation the textbook is trying to describe.  


Imagine a third situation now in which a RBC is at equilibrium with a solution.  If you just added urea to the solution, it would not affect osmolarity in the long term for the reason you describe.