Everybody is signalling, especially the people who think they aren't. We could sit here all day and game out all the possible interpretations that could be made from anyone's appearance, with respect to who they actually are, and it won't change much.

My take on it all: Programmers and other hot shot types often eschew formalities and conventions for dress and such, as a way of asserting status. "I'm professional and important enough to assert that my preferences supersede the ordinary" is what they want to signal. Of course, some are just childish enough to insist that dress codes don't matter in the slightest, and everyone must put up with their goofy graphic t-shirts. Others are willing to tolerate that stuff because most programmers are not customer-facing. But they still look like adult children when they insist on that crap.