Ok, so the main purpose of a blockchain is to get a bunch of computers that don't trust each other to agree on who did what when.

A blockchain gives everyone a "voice", so they can share what they heard. But it can't be the sort of voice we have in the real world, because one computer can easily impersonate a million computers. In the case of Bitcoin, your "voice" is your computing power. Your computer might be able to impersonate a million computers, but your CPU cannot do the work of a million CPUs. So it is nearly impossible for single malicious computer to "drown out" all the other computers and insert a false message into the blockchain.

Bitcoin has a limit to the number of transactions per second because it wants computers to pause and talk to each other before validating a transaction. The delay is a feature, not a bug. This arbitrary limit is designed to self-adjust, so adding more computers won't make the process go faster and removing computers won't slow it down.

That said, computing power uses a lot of energy. Too much energy. So now there are other ways to assign voices that don't rely on raw computing power but still prevent impersonation. This particular messaging protocol uses one of them, it is not based on computing power. But there is still a blockchain that contains everyone's messages, and a malicious computer in theory cannot overwhelm the common consensus. So you can be pretty sure that if a message appears in the blockchain, then it was sent by the person who claims to have sent it.

Note that unlike Bitcoin, this blockchain is not public. It is not like a secret agent placing a classified ad that all can read (but not understand). Even the encrypted form of the blockchain can only be accessed by servers whose users participate in the conversation. If your server has nothing to do with a group, then you cannot glean anything at all about the conversations within the group.