That’s the “objective” according to https://www.fide.com/images/stories/NEWS_2013/FIDE/Proposal_....
This is nothing more than a dumb outdated rule. He wore jeans. Not a fluorescent jump suit.
Clean jeans are normal for “smart casual” now. And the dress code shouldn’t be so prescriptive. If you can pass as “smart casual” somewhere else, should be legal.
There are people who think that it can be reformed and there are people who don't think that FIDE can improve, but certainly no one argues that its past proves its commitment to professionalism.
I'd say you'd be hard-pressed to find a worse performing global sport federation.