Yes, EPAS uses the motor to self-center the steering wheel, on top of caster effects. This is stated in this
Jalopnik BMW suspension article (search for "centering" in the article) and I assume applies to other manufacturers as well.
For your specific case, I wonder if the disagreement between the steering wheel angle sensor and the vehicle yaw sensor led to a "panic mode" that is not representative of the EPAS normal operation mode. The fact that the self centering force was the same across speeds makes me suspect this, as EPAS assist should drop with speed, and it makes sense to do the same for self-centering.
Finally, your concern about "pure" steering feel is a little exaggerated - any power steering system decreases the "pureness" of the steering. The only way to get pure feel (i.e. only suspension geometry effects) is fully manual steering. It's better to look at the final result instead of caring exactly how it was achieved. So far the consensus on the Twins' steering is that it is among the best EPAS and compares favorably to good HPAS.