Any time we've been to any of the Sig. TS's in warm weather, we've seen many men in long shorts (some even in short shorts), and in cooler weather, plenty of people wearing "nice" jeans. IIRC, no tank-tops, "workout" clothes, or pool attire are allowed (although one time in Jiko's there were folks who seem to have "jogged" in from the parks

). Either Disney has relaxed the rules somewhat, or they're not as stringent as many people think.
Maybe it has to do with the
feel of the resort; the YC seems to many folks to be more formal in atmosphere than the AKL or WL, so that may account for folks dressing up for the Steakhouse. IIRC, the GF Sigs. have the same effect on the diners' attire.