The data sheet for the ADF5002 shows an output structure with two 100 ohm internal pull ups.

I assume that there are open collectors or open drains connected to these.

The general description then states that the ADF5002 has 100 ohm differential outputs.

With the output frequency low, such as Fin = 4 GHz, wouldn't 200 ohms differential be more appropriate to maximize Po?

This question ignores effect of the 1 pF DC blocking caps. Thank you.

Datasheet figure is correct with 100 ohm pullup on each RF output => 200 ohm differential

Spec in data sheet then refers to 50 ohm and 100 ohm differential as this is typical interface that the part sees, such ADF4106 PLL etc.

So yes it is really 200 ohm differential