I'm still not getting your point. So what if software is designed with a goal of being magical? The question is how good the real-world results are.
From hearing them talk at events and reading their blog, they seem to be doing fine. Surviving for 6 years and their recent C round from a number of smart people also suggests they are doing fine. If you have some substantial indication that they aren't, I look forward to reading it.
The point is simple: The fact that they raised an investment round and haven't gone out of business is not an argument for their methodology.
"Fine" doesn't mean anything in that context. I could ship lower quality software to our customers and do "fine", but I prefer to ship high quality well-polished software that doesn't foist the cost of my development laziness onto our customers.
They've been in business 6 years. I'm arguing that these methods are sustainable, so yes, them sustaining them is a good hint that it's sustainable.
I also prefer to ship high-quality software to customers. And from everything they've said, so do they. Your (unsubstantiated) claim appears to be that longer feedback loops (with particular supporting practices) result in net higher quality than shorter ones (with particular supporting practices). I don't think that's true, and places like Wealthfront and Etsy are good counterexamples.
As for "doing fine at it", that they exist doesn't prove that, or even define what "fine" is.