These are actually separate issues. TPB isn’t legally responsible for theft if someone leaks an unreleased song. But, the publishing company can sue them for damages related to lost income based on TPB’s distribution of content.

Are platforms responsible for the content their users post? No. Rules for unusual situations (CSAM, terrorism, copyright, etc) have been established for probably decades by this point.

An online chat/message board is not comparable to an image generator because the company is not creating potentially harmful content.