No, you are mistaken. Most media is compressed, and most codecs have many options. But, unless you go around converting formats left and right and playing around, there's no impact on deduplication (which always sucks, by the way), because the same file, compressed the same way, with the same compressor, will be identical.
Compression, on the other hand, is a very different matter. ZFS implements lossless compression schemes, for obvious reasons, which is inherently limited in how much can be gained. Media is typically compressed in a lossy, but acceptable manner - for much better compression ratios - and lossless compression is normally already a part of that process. So, compressed media is unlikely to benefit from additional generic compression. However, LZ4 is safe to have enabled in ZFS even then, performance-wise, which is nice if you have mixed media and other data in the same dataset.