That's because flat structures are often, or often turn into, "flat-in-name-only" structures.

I don't think the Tyranny of Structurelessness is arguing in favour of hierarchy, or against other forms of organization than hierarchy.

I don't think it's arguing against "flat" or "anarchy" style organizations either.

In essence, I think it's asking us to do whatever we're doing better, more honestly, more effectively, and less stressfully. By acknowledging, clarifying, communicating, and seeking to understand the real operating structures, what's really going on. And then to improve them, using that understanding.

An actually flat organization might be good, I don't know. I've never seen one. I've been in some that claimed to be flat, and became stressful places to work, for the same usual reasons hierarchies can be unpleasant, including incompetent bosses (not called bosses). But I've also had some pleasant experiences in flat organizations, and I prefer it that way, if it's designed and run well.

You don't need strict hierarchies, necessarily (in fact I don't believe 'traditional' hierarchies are in practice ever actually ironclad: the org chart is only ever an approximation of the real power structure). It's more that you should plan your power structure carefully (many forms are possible!), and ideally make it as transparent as possible, as pretending that you won't have one at all is merely an illusion (you will never completely succeed! Firstly because power structures are, in their full glory, ludicrously complex and ever-shifting, but also because hidden information is itself a form of power)