Reviews require purchase, though. So for paid apps, it's much harder to do as a third party because you'll pay full price, whereas the developer gets back 70% of what they pay.
Although I have no idea how you'd create dozens of accounts with independent payment methods without it becoming suspicious. I guess you can use gift cards anonymously but that's almost certain to trigger even the most basic anomaly detection.
You're assuming that Apple has no process to distinguish between intentional fraudulent reviews coming from the developer in question, and random fraudulent reviews from others. But that assumption is not supported by any facts.
There is no evidence it isn't this either. Detecting fake reviews is easy, detecting the source (probably a click farm) is probably pretty easy too but getting to the bottom of who hired the click farm would be extremely difficult. You'd probably need assistance from law enforcement or a court, probably in a developing country, to get that information from the click farm operators.
I would think its pretty easy to hire a click farm to blatantly post fake reviews to the app store. The worse the click farm is at covering their tracks the better if you're trying to get a competitor banned by Apple.
Dash was a popular app among developers who use Apple products I doubt the fake reviews would be needed. I wouldn't be surprised at all if this was caused by a third party.
Without some direct information from Apple, it's perfectly valid to wonder if they were manipulated in some way. There aren't any facts available to us that point in one direction or the other.