To expand on this a bit: he gives the following supporting example:

I once wrote an article about salary negotiation. If you go by the numbers, it created more value for more people than any other single thing I've ever written. (I keep a label in Gmail for when folks tell me they got a raise as a direct result of advice in there. The running tally is in the high seven figures a year these days.) I think if I were to revisit the topic today I'd write substantially the same advice. However, that article has a date on it, just the fact of it having a date on it makes it less useful.

I have seen variants of the following conversation happen on Twitter / Reddit / HN / etc multiple times.

"I just got a job offer as a front-end engineer at a Valley company. How do I handle the salary negotiation?"

"Patrick wrote about that here. It is good advice."

"That looks like it was written in 2012. Do you have anything more up-to-date?"

History is a pretty wild rollercoaster, but nothing which happened in the interim has suddenly made "Don't negotiate your salary!" or "If you do negotiate your salary, start by naming a nice low number. You can always work your way up later!" into good advice. And yet if you put a date on your work, people immediately assume it gets stale.