It doesn't require perfect removal of the original film grain. A mishmash composed of remnants of the original grain combined with new synthetic grain can still look correct to our brains.
According to this paper, they're going a lot of effort to reproduce the original grain very accurately. If they combine that with some half-assed residual grain from the original, why go to all the effort to faithfully reproduce it? Based on your assertion, it wasn't really necessary to begin with.