Would you insist that you should be able to "make" the moped? Nobody will be as self-sufficient as the early Neolithic people ever again. For me, "making" things by building them from mass-manufactured components such as solar panels is perfectly fine. People don't shy from installing solar panels just because they can't manufacture them themselves from sand anymore then they shy from buying electric motors or CPUs.
The perspective I get from the article is of decentralized self sufficiency. Being able to harvest methane in their locality, and use that as fuel is quite appealing versus purchasing and installing solar panels or relying on power lines.
I’m not saying your math doesn’t check out; rather, the author is going for a different set pro/cons than just pure “efficiency”.
I bet you'd still be able to do this in a local municipal biogas plant more efficiently. It would also scale better should you wish to run more than several mopeds like this.
Again, that’s centralized. Less central than your previous example, but still requires a dependency on their municipal plant and power lines. Not sure why this is a confusing concept.
The concept is not at all confusing, it's just useless. For load balancing, efficiency, and reliability reasons, a microgrid is more desirable than isolated households in pretty much all circumstances.