They stated that at the time they decided to remove review bombs, they defined it as “an event where players post a large number of reviews in a very compressed time frame, aimed at lowering the Review Score of a game.”, since there were no examples of one “positive review bombs”.
According to Valve, what happened to AC: Unity seems not to be a case of the review bomb as actual players increased alongside the reviews and most of the reviews looks like standard reviews of a new player, or a player that’s returning to a product they bought a while ago.
And about if it is off-topic or not, they said they’re not sure since context can change. Mentioning that if there’s a game that heavily focused on Christmas, they “suspect we’d see it have Review Score fluctuations around Christmas-time”.
After Ubisoft funded €500,000 and gave Assassin’s Creed Unity away for free according to what happened to Notre Damn, the game received hundreds of positive reviews, which was enough to change it’s “Mixed” rating to “Very Positive”.
Review bombing is review bombing, recent positive reviews in Assassin’s Creed Unity is off-topic and has nothing to do with the game it self. Therefore, it is not any different to Borderlands 2 reviews and Steam should remove it to make their system consistent and neutral. If they doesn’t, it is not any different from biased censorship.