During my first startup as the CTO I made the mistake of thinking this way. I was so wrong I burned through 2M like it was nothing.
The "product" of a startup is a sustainable business model that can scale fast. The core technology may or may not be a key element part of that model.
So they solved a customer problem with interesting technology - regardless of arguments if it ACID compliant, or as good as <replace with whatever you think is "better">.
Where they sustainable at the beginning? No.
Are they sustainable now and did they scale appropriately? Yes.
As a startup that went through it stages, they did it. This is not easy because it requires all the elements to work properly (market need, sales, marketing, engineering, operations, etc).
So what if RethinkDB/or_other has a feature that is better? Irrelevant.
Even better they did this ethically, which is more than I can say about other "startups" with insane funding and dubious and questionable strategies.