True, most agency companies may make money but their products definitely have quite a lot of room for improvement, since the founders are not engineers by trade and thus there is no permeation of engineering culture as in other tech companies. However, I will say that it would be good for tech people to learn the marketing side if only by doing it semi-professionally for a client, just to see what goes into it. Then, they can choose to bootstrap or go the VC route, not necessarily create a whole new agency from scratch and continuously work on that. It's simply a learning method, one where you conveniently get paid to do so.

Yes, I don't think content is the only way, and I don't believe most of their growth was from content, anyway. It seems like it was just sending out lots of messages to people and hoping they're interested. Content marketing is a secondary mechanism, it is inefficient to get you your first customers, and it is definitely not the best go to market strategy as you say.