I would not be confident about it applying to Italian passholder reservations as these are effectively a payment directly to Trenitalia for their fee. I also can not find any terms for the offer to say anything on this.

Perhaps the best thing to do would be to ask OBB directly and see if they can give a definite answer one way or the other.

A few drawbacks:

The date of reservation and the date of travel needs to be in the validity period.

You can’t book reservations for others, just a single one for yourself, so every member of your family would need to pay 24,90 € to get the reduction and needs to do a reservation by itself (maybe getting places far away from each other).

You can’t buy the reduction for persons under 15 (if you are going with children).

Technically you don’t buy a seat reservation onlline, you do buy a reduced ticket when buying a passholder reservation for Italy online so I don’t think this will technically work anyway. 

And you need to open an ÖBB account. 

But it’s possible to cancel 14 days from the date of buying.