The frum world tends to think about tzniut (modesty) in terms of skirt length and collar buttons. By that measure, Chelsea in her sleeveless shirt would hardly be held as a model of tzniut. She'd be looked down upon in many circles as crude and immodest, and in fact she'd potentially be in danger if she walked through certain "modesty-obsessed" communities in Israel dressed like that.
And yet Chelsea's whole demeanor bespeaks tzniut of a much more profound variety. She walks without the slightest air of self-importance or ego, without vanity or sexual provocativeness. She encounters gifts, appreciation and attention with a distinct feeling of awkwardness, a sense of "Me? I don't deserve all this." And that, I submit, is a sign of true modesty.
Compare that to the (unfortunately not small number of) frum Jews who walk around with the sense that the entire cosmos revolves around them, with a feeling (and indeed a whole religious philosophy) of superiority over non-Jews, a sense of arrogance in the absolute certainty that they're fulfilling the will of the Creator, whereas everyone else basically doesn't have a clue, is at best ultimately meant to serve them, and at worst is subject to Hashem's "wrath upon the nations". I don't care how many frocks or hats or sheitels you have on or how many buttons you have buttoned - if you think you're "the stuff" and everyone else is just "chaff", then not only do you not know what real modesty is about - you've turned the very concept on its head.