No. To maximize usage, sales, margin, etc. you would want to sell as many vehicle combos as possible. If FDU was limited but you had available capacity in RDU then, utilize it.
Typically these type of production lines would be universal in the sense that they could run either unit that you needed with a simple changeover. But if you have upcoming demand for something that could get you more margin, then you want to shift more capacity to the higher margin item. So you would sell a variant that allows you to still move units, but net higher overall output of total units.
I read this as:
Y is launching with dual-motor/P configs that cost the most. So every DU that can put in a more-expensive Y (and still manage to sell a LR RWD 3 too) the better.
Though I'm a little surprised to hear Tesla has enough spare battery capacity to make BOTH of those cars that front DU is a constraint- last I knew batteries were the primary supply constraint on making "enough" cars to fill all orders.