Better question is why don’t both use a dot like URLs do because people have had 30 years of getting used to that:

username.lemmy.world

Plus this wouldn’t get confused with an email address.

The bigger question is why Matrix didn’t follow the long established standard for this, which is using @.

But I guess it fits into the general pattern of: Monolithic, Awefully Trendy ReImplementation of XMPP (MATRIX).

P.S.: it’s nice to be able to have your ActivityPub, XMPP and Email all on the same address 👍

Personally I like the @ way more. You even read it as “at”, which makes sense in this context. “Gerryflap at feddit.nl” instantly makes sense. It aligns with email, so it also makes it easier for newcomers.

I am no dev of lemmy, but traditionally, to connect remotely to a computer it was user@computer.name. The mail addresses simply use this pattern. It’s nice for Lemmy to use it, and I’d say ot would have been nicer for matrix to use it (not that I really care)

Because Lemmy is like social media and Matrix is like Slack and Discord and they both followed the conventions of their predecessors. You’re welcome to go take it up with the devs on Lemmy.ml.

@username(matrix.org)

It looks like it’s a Webfinger implementation to parse the ActivityPub user URI. https://news.ycombinator.com/item?id=35750312

The @user@instance.tld format has become a convention across most if not all ActivityPub platforms. In the end the software will look up https://instance.tld/user anyway.

• #room:matrix.org this one won’t be possible. # is used for tags/hashtags in mastodon, one of the largest fediverse software. Currently only mbin and piefed support tags. PieFed will work on mastodon’s support when API work is over. So the # may create problem later.
• @user@server why not ? We use email example to explain federation. But we could change with @ to : because we use @ for mention in lot software. :)