I think there's a big difference between "read once" and "know". When some people say a fact is "known" they mean strictly that you are able to recall that fact in toto. That is probably overkill for a Web programmer.

After you read something once you don't strictly know it. However if you run into a related problem down the road, you have a good chance that you can remember enough of what you read once to put a Google query together and find the original article.

After thinking about it a little more, what they probably mean is that while most programmers will forget most of what they read here (because they don't use it day-to-day) it is reliable information that will help shape their intuition about how memory works. That is probably what they mean about every programmer and this article.

Every programmer ought to read and test their intuition about how memory works against the content in this article. If they are surprised by something they read, they have the chance to adjust their intuition so that it is closer to the truth.