I am not sure about it as in this case the user was asked to promote the brand/project by the project itself so he did not claim it as his own content and also mentioned the project's URL which means he posted the content for the company.
Posting content written by somebody else without providing a source is still plagiarism, regardless of whether they gave you permission to do so. You are passing someone else's words off as your own.
If they used it as a quote, or included a link to the original, then that wouldn't be plagiarism. However, if there are dozens of bounty spammers all posting the same message, then there is a good chance they could be banned for spamming, rather than banned for plagiarism.