This is how myths get started. He said it and then I 'passed it on', because if it came from him so I assumed it to be true, 'cause he's a pretty swell guy, rinse and repeat.
In other words, it's an argument-to-authority (discarding the Latin here). Claiming "some famous guy said it!" is no more a proof than claiming "crazy, homeless Bill said it!": a convincing argument should be able to stand on its own weight, without being stated by a credible source.
If the hypothetical 'crazy, homeless Bill' said that the sun were bright, he'd be correct regardless of the fact that it came from 'crazy, homeless Bill'. If Ben Horowitz stated that 1 = 2, then he'd be wrong, regardless of the fact that he's 'Ben Horowitz'.
Argument from authority is a formal fallacy, but in terms of practical reasoning it's a valuable tool. If you find that someone has been reliable in their claims in the past, you would be a fool to not give some weight to future claims. Of course, that has to be modulated by your priors for each claim. But it is perfectly reasonable to form a weak belief on a respected person's say-so.
