[Lurking Libertarian]

I suppose it isn't, strictly speaking, self incrimination if you require co-conspirators to testify against each other... The end result is indistinguishable from it, though.

He would have had a right to assert his privilege against self-incrimination, because testimony against his co-conspirators could have implicated him as well. But the Government gave him immunity for his grand jury testimony, so he has no 5th Amendment privilege.

[null and void]

Yep.

Give Johnny immunity to rat out Billy and Freddy.
Give Freddy immunity to rat out Johnny and Billy.
Give Billy immunity to rat out Johnny and Freddy.

Convict Billy on Johnny and Freddy’s testimony.
Convict Freddy on Billy and Johnny’s testimony.
Convict Johnny on Freddy and Billy’s testimony.

Explain to my how the end result is different than if they had been compelled to testify against themselves.