On the other hand, I think I might know "why we think that": If hiring is the hardest and most important problem facing most companies, which people keep telling me it is, then it pays to hire with an eye toward further hiring. You might therefore put a premium on hiring people who engage with others via writing and conferences, not because these are necessarily the best widget designers, sprocket debuggers, or gadget platers, but because you expect these people are more likely to go out into the world and be noticed by the great designers, debuggers and platers who you'll want to hire next.
Obviously this can be overdone, and if you feed this process back on itself you'll end up with a one-dimensional clique of a company - in part because blog posts and developer conferences are a very narrow and biased sample of the universe of competent and potentially-competent people. But it shouldn't be surprising that companies think that "always be advertising" is a characteristic of a "top engineer": The problems of hiring are at the top of management's mind, and are probably the only thing at the top of upper management's mind - in my experience, the CEO could not care less about the glitches in the firewall-configuration system, but she interviews half a dozen candidates every day. Every all-hands meeting is filled with exhortations to blog more, shake more hands, hand out more cards, meet more people.
The best developers I've seen in terms of producing awesome technology tend to be quiet, methodical people. They are highly disciplined, and have a routine.
This regular, structured way of doing things means that they consistently practice out of hours, commiting regularly to an open source project while mere humans like me flit around playing with new technologies. They develop expertise.
Would you want this kind of developer evangelizing your technology? If you sat them with a client, would they have the soft skills to drive the conversation where it needs to go? Would they enjoy, or avoid these parts of their work?
I think people are people, and people are different. It's worth recognizing this and looking for the sort of person you need.
It's $INGROUP bias, and sectionalism. On the one hand it's self-affirming ("I go to conferences, I keep current on Hacker News, these are things good developers do, I'm a good developer"), and on the other hand it's trendy and convenient ("I'm going to post my job ad on Hacker News because that's the trendy place to be for good developers").
I think it's ultimately an echo chamber effect. I'm guilty of associating Hacker News with talented developers, but I'm aware of this bias and it's because I pay attention and follow the posts of people who have more or less proven themselves.
I think it would be very difficult to validate these assumptions.