The datasheet gives an "open loop" output impedance of 375 Ohm, to me that seems high, unless I have lulled myself into believing op amps are more ideal then I though. I expected something in the 10s of Ohms. Also Wouldn't an output impedance be gain dependent?
On a non rail to rail opamp, you can expect an output stage like the one on the left:
This emitter follower topology has rather low open loop output impedance. However, it is not rail to rail. The output can't go higher than Vcc minus one Vbe, minus the top current source's minimum voltage.
The schematic on the right is rail to rail, but then we have a common emitter output stage, and the output is taken from the collectors.
This means the opamps' output is essentially a high impedance current source. Its high impedance is kept in check by an internal local feedback/compensation circuit, of which the only part shown here is the capacitor. Due to the capacitive coupling, this scheme does not work at low frequencies, which explains why the OL output impedance of your opamp rises at low frequencies.
So, yeah, without special circuitry, a rail to rail opamp will have higher open loop impedance due to the topology of its output stage. It counts on feedback to lower it.
Low OL impedance in a rail to rail opamp indicates the designers employed some clever local feedback around the output stage to improve performance. It's a compromise wrt cost, idle current, etc, as usual, can't have everything.