Mini with EQ on flat sounds differently compared to Mini with EQ disabled.
For transmission error DAC can decide to replace missing sample with the previous one.
The first point is not relevant. This is about, for example, Mini with EQ ON sent over optical vs. coax S/PDIF; or conversely about Mini with EQ DISABLED over optical vs. coax S/PDIF. Point is: we are talking about when the exact same set of processed bits are sent one vs. the other.
The second point is possible. But generally, that will weigh in favor of *coax* being more accurate. For a lot of the cheap TOSLINK optical cables people buy, they are made out of very cheap plastic and often get are bent with tighter bends than the specifications allow for (bend radius that is too small is bad for the glass and/or plastic). Also, people may try to run the optical TOSLINK cable over a longer distance that it is rated for.
So basically, don't listen the the person.
Oh, and by the way, the issues of jitter, packet loss, and latency are vastly higher on the network-side of the streamer. Provided your optical cable is decent, your cable length isn't too long, and the engineering in the Wiim is solid... then any bit loss or jitter on the optical vs. coax side won't matter.