"Bart@nerdster.org"
is Bart.
We see "Bart@nerdster.org" because
Bart used the Nerdster to sign and publish the follow statement.
(Same dude as "Bart@ebay.com" or
"Bart@discord.com" would be.)
"Milhouse" is Milhouse (simply, with no @domain) because that's who Bart wants to follow: a person, not an account.
It distributes people's identity keys using statements signed by our keys, and so this can't be forged.
"Milhouse" and "Bart" are not available usernames that some users
grabbed for their accounts at nerdster.org.
They are who Lisa's network says they are, backed by a complete,
cryptographic chain grounded by Lisa's own identity.
The Nerdster follow statement above is not part of the identity layer,
rather it leverages the identity layer.
Examples:
Marge rates a recipe,
Bart follows Milhouse.
Here are the same statements shown in the "Vouch, Delegate" box, but this time interpreted by the Nerdster from Lisa's Point of View: Trust, Block, Replace, Delegate, (Delegate and revoke).
(On the Simpsons Bot Farm, Lisa scanned her mom's phone, Marge scanned her kids' phones, and Bart scanned Milhouse's phone.)
I am who your network says I am, across all services.
You own your crypto key pair. It's yours, you own it. Use it to sign
whatever you want.
The public component of your own public/private cryptographic key pair
is your identity.