If your going through abu dhabi worth checking hotels with this deal against other sites, from the hotels I checked it works out cheaper, looks like anyone can use this deal not just those flying etihad
eg (sept): Grand Millennium Al Wahda
Deluxe Room with breakfast 2nights US$94 + 30AED Municipality fee = AUD$136 vs otel.com $217 or $182 without breakfast (checked cheapest through hotelscombined.com.au)
Book a two-night stay in Abu Dhabi and get the second night free! Choose from over 60 hotels as well as 2-for-1 deals on a wide range of activities.
Hotel rates starting from only USD44 per person / room!
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Yes it is for only those flying Etihad as when you go to complete a booking it has a pull down list of Etihad only flights for the day for you to choose which flight you are arriving on. No Etihad flight no free night.
It's also in the T&C's.