All of the knowledge maps above have the same area. Someone that is smarter or works harder to learn can increase their area, in terms of depth and breadth, but most of us are never going to fill in the square entirely.
And that's ok. In fact when a client hires you to do something for them, it's not usually because you know their industry better than them, it's because you can do something they can't and it's cheaper/better/faster to hire you in to do this one thing than it is to try to develop the capacity to do it in-house.
A lot of professional services firms tend to have a comparative advantage in knowledge of particular techniques, that can be applied across a fairly wide range of industries. For example, tax planning, financial modelling, business strategy, legal drafting, technical writing, project management, and so on.
The best consultants I know are very clear with their clients in what they know, and where the limits of their knowledge lie. This helps both client and consultant work best together to identify potential blind spots that one or both parties may have, to focus attention there, and have the best chance at coming to the right decision.
The key thing that creates value in team building is complementarity of skill sets. The problem for pure generalists is that when they're working within a single domain, a lot of their knowledge is either not related to the domain, or duplicated by others in a team. A pure specialist, at least can add a lot of value if deployed correctly.
So, to answer the question I pose in my title: how much do you need to know about a field to give advice about it?
The HBR article's answer is something like: Not much. You're a generalist manager, don't waste your time duplicating knowledge that exists elsewhere because you don't want to look ignorant and ask a question. Rely on your specialist minions to deliver and get out of their way!
I think that's pretty solid advice, but I might add that you need to learn as much as you need to know how to leverage your specialist knowledge in the field. If you are truly the "leader without expertise" that the HBR article mentions in its opening paragraphs, then you'd better develop some expertise soon, or you're probably not long for leadership!
So, in conclusion: know thyself! Are you a specialist that needs to generalise, or a generalist that needs to specialise? The right answer will depend on you, your personality, your abilities, and the career and life you envision for yourself. Finding the right balance for you will give you the best chance at developing a satisfying career working with, and managing others.