You need to have played the game or owned it to review it on Steam. So for a game with a price tag you have to buy it (on steam, elsewhere, or someone bought it and gifted it to you) to be able to review it, meanwhile free games you have had to added it to your library to be able to review it. People can refund a game after reviewing it, but it also states in their review that the product was refunded.
Not sure if you can buy and then refund an expansion like DawnTrail (never tried it myself), but even if you can, anyone buying it just to up/down vote and then refund will be obvious if they review it on DawnTrails page.
Steam Reviews page: https://store.steampowered.com/reviews/
Q. Do I need to have purchased a game in order to write a review of it?
A. Purchase is not required. You may write a review for any product in your Steam account. Whether you registered a Steam key obtained elsewhere, or played a free game, you will be able to write a review for that product. Your playtime in that game or time in that software title will appear next to your review.