It basically seems to advocate using functional programming, or at least writing pure functions for the code that you want to be testable.
Got a little bit disappointed in the content of the article. In fact, the whole article amounts to the three sentences of information "Write functional code. If it doesn't work, reorganize. Try it and let me know". I wish the author at least pointed how to convert the code to the testable state. Not to mention the "reorganization part" totally blows all the meaning out of all previous sentences -- if writing pure functions may not work, does what you say even matter?
OK, at first I thought this was just a bad programming language created by a person with no taste, but the more I read into it, the more I understood that it is actually a giant joke-programming-languages. I didn't find it too funny though.
The website actually recommends against using the same date for everything. I think that was the point of the website rather than giving a random date. Namely, promoting not using personally identifiable information such as date of birth. I suggest thinking of the random date display as a sort of practical joke.
Also you're right that no one writes down their fake date of birth -- that would be weird. As an advice that wouldn't make sense, but what if we consider the case where people "don't care" and always chose "default" date of birth like "Jan 1, 1999"? That would be the same on all websites.