There's a lot to unpack here, but the general gist is that McKinsey is overpaid for mediocre services. It's hard to argue against that, and McK in particular does have some insanely unjustifiable rates, but I do think the author has a bit of a misunderstanding of why consultancies are hired in the first place.
To be clear, when a consultancy like McK puts a 23 year old analyst on a project, the $50k/week bill isn't just paying for the knowledge in the analyst's head. The analyst is really just a vehicle to deliver the expertise of the actually-experienced consultants and knowledgebase of the entire firm. The experienced consultants don't have the time to sit and write out the powerpoint deck, so instead they throw as much knowledge as they can at the analyst and the analyst is the one who synthesizes it into something digestable by the customer. Is it worth $50k/week? Fuck no. But it is at least worth recognizing that you are getting more expertise than a sole bachelor's degree.
Second, as someone who has both worked as a consultant at major companies like McK and worked on the other side of the table, and as someone who has long despised the consulting profession because of its overpriced bullshit, I think the author would be surprised at how effective even a measly 23 year old can be. As a consultant, it would drive me insane that my company was billing me out at $700/hr to help implement an IT system when all I was actually doing was reading and regurgitating the software's documentation and making sure that the client didn't ignore it. Surely the client could do this themselves and save the $700/hr, right?
It took me a long time to realize that the answer to this question was actually no. The typical HNer will probably be surprised (and saddened, if you're like me) to realize that the average corporate worker bee is in fact not competent enough to do the most basic tasks, such as refer to documentation, without hand-holding. It is an unfortunate reality that sometimes a well-educated and well-vetted 23 year old analyst can do things much better than an 20-year industry veteran. Companies do know this, and this is what they are paying for, much to the chagrin of the rest of us.
As a last note, the author mentions the possibility of just hiring an IT professional to do the work instead of hiring a consultant. This is something I also see come up a lot in consulting discussions. The reason this isn't done is because hiring an employee full-time brings on a lot of risk to the hiring company. It can take months to hire someone, many more months to train them, and then if they don't perform up to snuff, it can take years to build up a case to fire them, all the whole they are sitting on your payroll taking up budget. A consultant, on the other hand, can be hired in a day, trained in a week, and if needed, can be fired in a minute. You pay a premium for the agility to hire and fire them quickly, but that's the entire point.