T is a frequent abbreviation of t-shirt. I assume you are right. T-shirts are usually made of cotton, not fleece, though. It probably refers to the style. There are both long-sleeve and short-sleeve T's.
But as you can see from the photo, this is something that's far from an ordinary t-shirt. I believe the term "t" in the context of clothing has come to mean something more like: "any collarless, vaguely t-shirt-like shirt." And the term tends to be applied especially to women's clothing.