From today the license applied to the project will be the Apache 2.0 license with an extra line forbidding usage of the codebase as an integration or app to Atlassian's Confluence or Jira products....
Atlassian could sell extensions, though, they would just need to comply with the AGPL. The AGPL means that the entire platform must comply with the AGPL, so proprietary platforms couldn’t use it but in a fair “applies to everyone the same” and not “we don’t like you individually” kind of way.
He should just use AGPL then.
That’s substantially more restrictive than “Apache but you can’t sell it through this specific channel”, and it wouldn’t help this particular problem.
It’s not that the knock off extensions don’t want to share their code (they probably do).
Atlassian could sell extensions, though, they would just need to comply with the AGPL. The AGPL means that the entire platform must comply with the AGPL, so proprietary platforms couldn’t use it but in a fair “applies to everyone the same” and not “we don’t like you individually” kind of way.