I do not know the specifics of this particular deal, nor have I used the product, but I hope the points are of going to be some use.
Let us look at it this way. Assume there are 200 funds out there who can do a series A of this size. That would naturally mean that not every fund is going to be either a leader or someone who spots new trends (there are not enough trends out there). Naturally, a lot of them have to invest in deals in other companies in a hot sector.
A lot of investment is momentum-driven and momentum is often driven by the narrative. You have to remember that as long as a successful exit happens, the fund winds up with a good deal irrespective of whether the public (IPO) or the acquiring company (M&A) eventually profits from it. NoSQL has that momentum at the moment.
A healthy start-up ecosystem can easily support more than a handful of companies in a single domain. Once the narrative for the domain really picks up, even the not-so-great ones (again, I have no clue about RethinkDB) stand a good chance of being acquired as long as there is decent enough traction and the sector is so hot that there is pressure on the GPs to make a play in it.
The later they get into the game, the pricier the ticket becomes, but you get lesser risk too.
And all of this is perfectly OK and fair.