The CEO of the Product
Horowitz opens with the frame the whole essay hangs on: a good product manager acts as the CEO of the product. Not because they have the authority of a CEO (they don't), but because they take the responsibility of one. Full responsibility, measured against the success of the product.
That responsibility has no escape hatches. The good PM owns the outcome even for things outside their direct control: the right product, delivered at the right time, with everything that entails.
A bad product manager, by contrast, has a long list of reasons. Not enough funding, the engineering manager is an idiot, Microsoft has ten times as many engineers on it, I'm overworked, I don't get enough direction. Horowitz's response to that list is cold: Barksdale (Netscape's CEO) doesn't make excuses, and neither should you.
“Good product managers know the context going in (the company, our revenue funding, competition, etc.), and they take responsibility for devising and executing a winning plan (no excuses).”
Key idea
The good PM takes full responsibility for the product's success and makes no excuses. The excuse list is the clearest tell of a bad PM.