I actually think that mastodon, scuttlebutt, and other platforms and protocols (ActivityPub, etc.) represent the beginning of an evolution/revolution towards more decentralization - ok federation in some cases. And, the next versions of these platforms will either begin to converge...Or, there will be yet a new platform that will combine the best aspects of each of these. I can imagine a platform that has a neat UI like mastodon, leverages ActivityPub while online, and scuttlebutt when offline, etc. I'm excited about the work going on around all of this...so for now, your question is valid. But eventually, there might be less and less differences.

By the way, to answer your question, both mastodon - and other platforms that leverage the ActivityPub protocol - as well as scuttlebutt are similar in that they're either federated or decentralized; basically do not need to rely on any centralized controlling entity. They differe in numerous ways, but the biggest would be that mastodon - and other ActivityPub servers -expect to operate online, while scuttlebutt takes the "offline first" approach. Sure, there are other differences like UI, etc...But that might be the gist you're looking for.

For me it is quite different to Mastodon as it's not just a twitter replacement run on different servers. It has entirely possible to run this completely without a server, so if in an isolated network, the local LAN could be your sole connection.

Then perhaps one person goes out to another town and connects to the internet, they collect many many messages from internet.

They then come back to the isolated network and connect, everyone gets the messages they collected. I have seen this effect happen when using both laptop client and mobile phone client.

When internet was down, my phone was able to get messages. Then it connected also to the wifi and my laptop (on the wifi) was not getting messages from internet; but got from my phone.