By design, the service doesn't provide any documentation; it provides references to existing technical documentation (of any kind, including blog posts).
Those who expect a focused introduction to each topic will find it very tedious or hard to proceed (for example, the SQLite exercise has important details buried in a very large and confusing webpage), and likely hate it; those who like the challenge of understanding loads of raw documentation will love it.
Providing a structured path and (automated) test means can be stimulating, and can make the difference between (deciding to) learning something or not.
Some people are certainly entirely autonomous, but at the very least, there is a spectrum of need for stimulation when approaching a topic of study.
Exercism is the closer service I can think of, but it's based on simple exercises, not real-world projects.
There are a few books that have a similar target (build X), but much narrower in scope (either a single language or pseudocode, and certainly no automated testing/team features).
This model is great for someone who loses patience with all the groundwork setup.
However, I do agree that the next leg is taking this MVP/Prototype level to production and ideally sell it as a real alternative to the commercial version of Redis.
Great idea, but only accessible to the rich.
I think this is a bit much. I wouldn't buy it, but I think this is deep technical content that you might not stick on forever, and they need to pick a price for it. If it's too high or low they'll find out soon. No need for any other inputs than that.
*Saw in sibling comment this is recent addition. Without it, yeah, it may be a bit expensive for one person.
What is still missing, in my opinion, and is badly needed, is content or even an idea on how to teach taking such projects from toy prototype version to the production quality one.