Line out impedance is 100 Ohm unbalanced, 150 Ohm balanced.

This is not “so” high, and necessary for the output stage to have a good sound.
This way ADI-2 FS can drive very long cables (up to 400 m) without the output stage developing instability.

The headphones out has an output impedance of 0.1 Ohm BTW.

This is the paradox:
You need to use lesser, slower OP-Amps to run them with lower Ohm de-coupling.

You wouldn‘t pack a heavy load on a race-horse.
But an ox can carry it – although not very fast.

Output impedance isn’t a sound quality criterion on preamps, as long as it’s within practical limits.

In extremo: ADI-2’s line outs can drive any load, even could be mis-used as headphones outs (I tested this).
Lower Ohm outputs would completely fail under those conditions.