Do I really need the open loop output impedance of an op amp?
Open-loop output impedance of a opamp is rarely, if ever specified.  However, the maximum current source and sink capability usually is.

You should therefore go with the datasheet and consider the opamp output a current source within the maximum specified current capability.  It's impedance should therefore be considered infinite, since that is the impedance of a perfect current source.

In reality, the impedance won't be infinite, but you don't know what it will be.  CMOS output opamps probably look mostly resistive, but the current source model may actually be closer for bipolar output opamps.

In any case, don't try to read into the datasheet what it doesn't say.  You are only guaranteed what it guarantees.  Assume the current source model when designing the circuit, and everything should work.  If you design the circuit right, the open-loop output impedance will be irrelevant anyway.  In the closed loop case, the output impedance, like the gain, will be governed by the feedback.