Yep--the only real "rules" are that very short list of what is actually not permitted in the dining room.
Shame on the CM who apparently made up their own rules and told the OP what they heard. Wrong information. And that CM might just be an operator at a call center in Kansas.
The dress suggestions are clearly posted on the Disney Web Site.
You are ASKED to adhere to them. If you decline, then they give the short list of what you simply cannot wear.
A good example is "dress shorts"--I'd say over half the men in shorts are in cargo or even military style shorts. Since they are not on the not permitted list, they are fine in the restaurant despite the recommendation for dress shorts.