In modern enterprise cloud software the “stack” is all your layers of client software, libraries, REST APIs, server software, databases, virtualizations, etc. Like many terms in computer science, “stack” has been overloaded so context is key. So in current parlance, a “full stack engineer” is someone who can work on client and server code, as well as understanding all the databases, caches, etc needed for the whole system to operate.
Not that Musk really understands any of this, of course. He just heard a word getting thrown around by technical people and tried hamfistedly to use it so he would sound technical.
