Great Article Jack!! Really enjoyed reading it :-)
I had been working in an Enterprise for a very long time 10 + years and before I switched to firm that specialised offering services to Start-ups.
In an Enterprise set-up, the roles/responsibilities of a developer are very clear. If you are a front-end dev, you are considered to be really good at being one. The firm wouldn't care for you to learn more as long as you deliver what you were hired for. Likewise for backend developers.
However, things were drastically different in the start-up scene. If there's an opportunity presents itself in an area which you haven't worked before, most often the developers too want IN on that opportunity so that they can acquire as many as skills as they can. The firm would also want you to pick up skills, to reduce cost, since it's primarily a start-up. It is a case of true symbiotic relationship. In my opinion, this was sort of the birth of 'Full-Stack' Developers.
But, the 'T' representation in your diagram, is very accurate. I would probably refer to the line that's vertical to represent your 'depth' in the technology and the horizontal line to be the 'breadth'.
In any case, I hope the hiring managers and HR is aware of what 'full-stack' really means. If not, I always have your article to share with them. Thanks for this great article!!