Yes, "well-known" is context dependent. But I would argue the WSJ chose a poor context. The reader already knows whether or not he/she has heard of Github, so the real question is "are they well-known in their industry?"
Pick any non-consumer-facing industry where the average person doesn't know the players, and everybody is "little known". That doesn't really tell you anything about their actual businesses or reputations.
I'd go even further, and say that GitHub is incredibly well-known within their target market (developers). Everyone else doesn't matter, as the business will not be built around the general public.
WSJ is supposed to provide good reporting with strong bias towards informing potential investors (as a financial/business news paper). The wording choice doesn't make a strong case for value associated with the company and sounds like some no-name co. just got a pile of money that they are like to lose. (An exaggeration on my part for illustration purposes only.)