There's a distinction to be made between lossless (compression) and lossless (bit depth/gamma/yadda yadda); typically the format conversion isn't considered part of the "lossy/lossless" distinction.

Definitely not something to be forgotten if you're expecting to compress lossless RGB, although this is pretty rare outside of niche-ish use cases (afaik game capture is basically the only place where people are likely to be using RGB and not YCbCr? could be wrong!)