You should interpret "programmers believe" not as literally thinking that this is true, but rather as "some systems are designed as if their programmers believed that" - and the latter is very definitely true; "Everyone has exactly one email address" is a very relevant falsehood, because there are systems with this baked in as a fundamental assumption.
I guess what I was getting at is, the earlier articles that this one riffs on were more along the lines of "don't forget to test these edge cases. If you don't, your system will break when people put in their unusual (but valid!) data".
To me, someone baking in "one email address per user" or "no numeric/symbols" feels different — they're not being caught out by tricky real-world data that they forgot to consider, they're just deliberately cutting a corner (e.g. our company naming policy is '[firstname][lastname]' so we're not going to bother supporting numeric input in the intranet input field).
Maybe not if you actually asked them, but you’d be baffled from how many systems are designed to require people to have, and use, exactly one e-mail address, ever. The programmers/designers of those systems did believe, implicitly, that everyone has exactly one e-mail address.
But that's just limiting the scope of development effort on a project to reduce time/cost. It doesn't mean that you believe anything you don't support is impossible to happen.
If an automotive engineer put a battery in an electric car that gave a capacity of 200mi trip, no-one would say "Engineers believe every road has a charging station at least every 200mi".
> You'd be hard pressed to find anyone who's at all component with the internet who thinks that this is true, nevermind a programmer.
On a related note, there are people without an email address who still use the web applications and smartphone apps that require accounts and/or notifications. In some (or most?) developing countries, people use phone numbers as the login identifier and may not have an email address (or not know that they have one and what to do with it).
> Everyone has exactly one email address
You'd be hard pressed to find anyone who's at all component with the internet who thinks that this is true, nevermind a programmer.
Maybe we need a 'Falsehoods writers of articles believe about falsehoods':
> You can just put any false statement in the list, even if no-one actually believes it and it will improve your article.