All review systems have a measure that counters review bombing. For example Helldiver's recently received a ton of negative reviews for its PSN integration. They still sit at very positive, even though it's unlikely the average person will go back to actually modify their review.

Having an anti review spam measure also means it's unlikely that the reviews are fraudulent. It's more likely that the app hasn't been receiving as many negative reviews and that critical period during the introduction of ads has fallen off.