This thing has 233k stars. I dare to say, without evidence, that the vast majority of devs does not know what e.g. the CAP theorem is——and they’ll never need it. The vast majority of “systems” are small-scale. Who’s this massive audience?
Also, to add to your comment about CAP. knowing CAP (consistency, availability, partition tolerance, choose two) can be useful when choosing which database to use for a production system, I.e. can you get away with a fancy distributed gossip database or do you need the robustness of a rdbms.
99.9% of the time a RDBMS is fine, but if you're vying for a senior staff role, you need to sound like you can talk about the 0.01% of the time where Riak or something like that makes sense as a primary data store. That's why this guide mentions it.
Personally I think it looks interesting and I work at a small web agency. We usually only work with a few parts of these kinds of systems, but occasionally we have clients like a large sports ticket vendor, where a few more of these patterns are helpful.