It's usually meant to describe devs as they develop and mature. They typically start with a strong descender, a specialty, then they branch out.

Generalists and T-shaped people are usually separated into different groups taxonomically.

I'm with you, though, I'm a generalist, a jack-of-all-trades, master of none. I like to describe the shape as "cave ceiling with stalactites," because there are several areas where I go deep.

I'd also like to point out that I have a few degrees, but universities tend to force people to focus, so being non-academic might make you more likely in my group. But as long as you own your education instead of letting deans and advisors stunt you (argue with them - they need that), academic education can be enjoyable and a way to experience even more knowledge broader and deeper than you might otherwise.