It was in the article.

He didn't explain why we can't make a "beefy" nuclear option. It's also the option I would have expected, especially when you think of nuclear subs.

I didn't think of nuclear subs. It would seem ideal for running a small moonbase. But I don't know how self-contained those reactors are, or how much they weigh. Surely a lot.

There are also the nuclear batteries in some of the longer lived space probes. Those batteries have the major advantage of being very simple - a lump of radioactive material that gets warm - attach a thermocouple and voila! Electricity!

LFTRs can scale down to closet sized, and most of the fuel (Thorium) is stable aside from the ignition uranium plug, but there's a LOT of materials to solve for long service.