Anybody can sell their books and electronics online (well, they could before Amazon started, yet amazon exceeded). Anybody can make clothes, yet Inditex is a monster. And why is there more than one restaurant. SF's claim is that their tech is a differentiator, and they are a tech company (unlike, say, Target or Blue Nile).
There are a couple of thoughts about "in store brand": their business plan may have required tighter control and visibility into the dimensions and materials of their product (dimension for the reasons discussed in the article and its footnotes; material because stuff that is tried on not in store and returned may need to be more durable, I don't know). It makes the plan more complex, but perhaps improves execution enough that it's worth the expense.
In the long run, if they really have a tech advantage they have the option of expanding beyond just in-house product or simply splitting into sub brands, or both.
I like stitchfix quite a bit, but I'm starting to become a bit disillusioned after seeing the small bag of clothes dropped at the Salvation Army which didn't fit or I didn't like and which it was cheaper to keep than return. At least I have a mortgage and this itemize deductions, so I can write those clothes off my taxes!