skeptical of any battery I'm not holding in my hand.
Um, yes. Still, this seems to be a reasonable way to go, especially since the developer claims to be able to make layers of carbon nanotubes cheaply.
Nawa, though... They announced in 2019 that they would be shipping ultracapacitors by the end of the year. But their products page doesn't list any. They're not a vendor known to Avnet or DigiKey. They announced a super-bike with ultracapacitor boost in 2019, and there are pictures, but none show the bike in motion.