I don't disagree with you, but I think you're missing the author's point. The author suggested these projects as a quick way to learn a new language or get familiar with a set of tools. Arguably, working on a more time-consuming product for customers is not the most efficient way to do this.
The author's suggested projects hone in on programming mastery, whereas your suggested project hones in on a plethora of skills. We can't really say that one substitutes the other as they achieve two different things. I wouldn't necessarily start a startup just to learn WebAssembly, but I would learn WebAssembly if I needed to for a startup. In the former case, my goal is to learn WebAssembly, whereas in the latter, it's to start a startup.
Not sure these are peojects to tackle with a new language. Simple repeatable projects like hn clones are better suited for those use-cases. More along projects to do in your favourite language bonus points for using language that isn't typical for the domain.
Like: A compiler in php or a spreadsheet in cobol or javascript console emulators.
The author's suggested projects hone in on programming mastery, whereas your suggested project hones in on a plethora of skills. We can't really say that one substitutes the other as they achieve two different things. I wouldn't necessarily start a startup just to learn WebAssembly, but I would learn WebAssembly if I needed to for a startup. In the former case, my goal is to learn WebAssembly, whereas in the latter, it's to start a startup.