Sharpening just makes the inherent aliassing more visible, and if that's an issue, the Sigma that designed their raw conversion software sound like they've not spoken to the Sigma that designed the cameras. RAW conversion and camera have to work as a team to "get it right". Even the Sigma adverts in magazines show nasty aliassing on the girl's eyelashes.
Aliassing on, say, perfect computer generated images, looks like your traditional stair stepping, but because real life has lots of detail that isn't in straight lines, aliassing from that manifests as almost a noise where you're getting uncorrelated pixels next to each other as neighbouring pixels are being fed widely different colour or tonal values. In computer graphics, they vastly super-sample the image to get a smooth, continous image that's not aliassed. To do that on a sigma, would require so many more pixels than it has that the size of the camera would be like a truck. Or you could slip in a small sliver of glass, an anti-alias filter which really is an engineering necessity, take the resolution hit, but get smooth, continous images.