Thanks for linking that. I haven't heard this Stealth hub, but it's probably quieter than the Ultegra hub.

The Ultegra hub isn't actually silent, but it's quiet enough that while actually riding on a road people just don't hear much. Freewheeling while riding up behind someone on a MUP, for instance, simply doesn't work to alert them. The Neos hub on my Stans Grail wheelset, on the other hand, can alert people that I'm coming from a good 20-40 yards out with just a few seconds of freewheeling.

If the OP is interested in hearing the difference between the Ultegra hub and a bona fide loud freewheeling hub, here's this little comparison. Note: the Ultegra hub is recorded from maybe a foot away. While actually riding out on the road you barely hear anything while freewheeling, and people further than just a few feet away pretty much don't hear anything at all. The sound of the tires rolling on the pavement is probably louder.