Well, the problem an index over multiple documents solves for is also not present in the application presented by the article. The plan for an individual trip (or even a lifetime of trips for most people) is not going to exceed a size that can be handled and indexed on the client.
You're right, the approach totally works for PKM apps with less than, say, 1 person*year of data, which is literally the first sentence I wrote at the top of this thread. (But – there are a lot of architectures that work with casual datasets! Like, store everything in a text file. Or fork bitcoin and run a full node on device, for that matter. What are we trying to accomplish here?)
