Maybe it is powered by a rainwater waterwheel, and thermal cooling is provided by inside out rabbit furs?

The idea of “off-grid” here is that you do not require any networking from your usual provider. You set up two devices with meshtastic and you can have connectivity over great distances.

For the decentralized part, I think they mean that you can communicate with any access point in an area without relying on a third party (i.e., make an account on their platform?).

So, off-grid means not relying on existing infrastructure and decentralized implies not having a central figure to relay your data. That’s what I understood from this at least.