Yes, with the Admin API key you can post as any user.
You can work around this by enabling the Do Not Display Discourse Name Field on the Publishing settings tab. Just enabling that setting on its own will cause all posts to be published under the name you have set as the Publishing Username. If you also enable the SSO Client option, users who login through Discourse will have their Discourse username automatically set.
If you are using WordPress as the SSO Provider and enable the Create or Sync Discourse Users on Login option, Discourse usernames will be automatically set.
For the case where multiple authors are intentionally publishing posts under a single username, there is a warning message appearing on the new post page that needs to be removed.