Let's dig down into the weeds here
From the article:
A single nuclear-diamond battery containing 0.04 ounce (1 gram) of carbon-14 could deliver 15 joules of electricity per day. For comparison, a standard alkaline AA battery, which weighs about 0.7 ounces (20 grams), has an energy-storage rating of 700 joules per gram.
From google:
One gram of Carbon-14 would be microscopically small, roughly equivalent to a tiny grain of sand, as a single carbon atom is extremely small, measuring around 0.0914 nanometers in diameter; therefore, even a gram of it would be invisible to the naked eye due to its atomic size.
So even 100 grams of Carbon-14 would be extremely small and generate 1500 joules per day for 11,000 years. I would think that might be adequate to power the mini cell phone size subs we are talking about.
Certainly upscaling this tech could power future spacecraft as well. and diamonds can be made artifically now so that is not a show stopper.