His vision of a "full stack developer" is a fairly reasonable one for software developers and engineers in general: specialise in at least one layer of tech, have at least a good understanding of the adjacent layers of tech, have some grasp of various layers of tech across the board, and also have a clue about business and legal concerns.
But by "full stack developer" other people have a different vision, i.e., one who is a specialist in all the layers of tech for which they would rather just hire one developer instead of hiring a specialist for each. Having someone who lacks expertise in some area cover it anyway may be necessary in the early stages of a new software house or when the developer needs to be an in-house generalist, but in the long run it may impede progress or even be harmful because developers simply cannot be expert in everything.
