Sound so simple but its amazing how many "entrepreneurs" are terrified of talking to potential customers and end up wasting a ton of tie developing things nobody wants.
Now, maybe you disagree that a graph with loops is that big of an error. But then you should write about the graph specifically. I don’t think you have grounds for disagreement with the basic principle of generalizing the quality of an article from part of it and using that estimate to decide whether to continue spending your time on reading that article.
My real issue with it are unlabeled axis - without them those graphs have no meaning except as an conceptual illustration of an idea that something loops around while growing. Also, OP assumed that x is time, which it does not have to be for the "graphs" to make sense.
 - see e.g.: http://en.wikipedia.org/wiki/Phase_space
tl; dt: nitpicking, and also picking the wrong nit.
But I actually like your point because its interesting (a little) to think about what the axes have to be for those shapes to work.
There is probably no confusion around the y axis representing traction - whatever metric is meaningful for the business at that stage. It could be users, or revenue, or engagement, etc.
That pesky x axis is the issue. In the second graph, it cannot be time, as many have pointed out. And that's confusing because the x axis in the first graph of the hockey stick shape is usually time. The x axis in the second graph could instead be a measure of learning, or perhaps product development as one commenter suggested. It probably works best as degree of product/market fit. As prod/mkt fit increases with iterations, so does traction.
Any non-technical, non-mathmatical person would look at that graph and say "yeah, you go around in circles for a while and then finally get things worked out" and I actually thought this was a pretty good post otherwise.
Which make the loops palatable.
"He was saying a real startup trajectory might look something more like this" (emphasis mine)
When we're graphing trajectories, not functions, loops are certainly allowed (see e.g. phase diagrams). Still, without labeled axis those graphs are pretty much meaningless, so I treat them more like an conceptual illustration.
 - http://en.wikipedia.org/wiki/Phase_space
When you first start off trying to solve a problem, the first solutions you come up with are very complex, and most people stop there. But if you keep going, and live with the problem and peel more layers of the onion off, you can often times arrive at some very elegant and simple solutions.
I wish I UNDERSTOOD (very different then "I wish I knew") that 2 years ago when I was starting my company. Focusing on the problem, discussing it with the people that actually have that problem and not jumping immediately to solutions but "peel the onion" is something very very very hard to do.
Hope that next episodes of your article will provide more info about HOW to peel that onion :)