> both it and the synthesized grain image look noticeably less sharp than the source

That's true, but at a given bitrate (until you get to very high bitrates), the compressed original will usually look worse and less sharp because so many bits are spent trying to encode the original grain. As a result, that original grain tends to get "smeared" over larger areas, making it look muddy. You lose sharpness in areas of the actual scene because it's trying (and often failing) to encode sharp grains.

Film Grain Synthesis makes sense for streaming where bandwidth is limited, but I'll agree that in the examples, the synthesized grain doesn't look very grain-like. And, depending on the amount and method of denoising, it can definitely blur details from the scene.