I think the parent's point is that it's absolutely feasible to build hardware and software that successfully enforces a DRM scheme, and also release the specs for the hardware and the source of the software.

That doesn't mean that a content producer will verify any old (modified) version of the software and allow it to play (and possibly "leak") their content.

Put another way: it's possible to build a secure DRM pipeline and then release all the source to it while maintaining the security of _that particular version_ of the pipeline. It's perfectly possible (cryptographically) to set it up so an unmodified version of that pipeline will be able to play protected content, while modifying it to (for example) dump decrypted bits to disk will cause the content to fail to play at all.