The web developers almost certainly do know how to do URLs.
Someone made a decision that this was the way they would look up the articles and this potential for abuse was almost certainly made clear to that person.
Remember, a place like the Independent has a lot of content to serve up, and they want it to be as efficient as possible. Verifying that the headline supplied in the URL matches what they put in the database doesn't help that.
I have absolutely no sympathy for the viewpoint that the web developers don't know what they're doing.
The boneheaded decision was how the Independent reacted. Leaving the loophole open and making a big fuss with
an editorial only publicised it further and made them look like incompetent idiots. Had they silently put in a simple rule that blocked just the circulated URL with a 404, or a redirect to the correct article URL (less good because it means the fake URL still "works") as soon as they were aware of it, the vast majority of people who have now seen the fake URL wouldn't have been able to use it. That would have given them time to decide on a more general reaction to fake URLs.