Personally, I think the issue comes down to how much code is enough and how to make a computer recognize the difference between someone throwing code tags around a word and a legit piece of code.
- False Positives: How frustrating would it be for a new user to post a link to a fiddle and also include a short block of code that perfectly replicates the issue, only to be prevented because "Links to jsfiddle.net must be accompanied by code".
- How much code is enough: Where should you draw the line between code and no code? 99.9% of the time a single word won't do it, but maybe it is a single line of code. So where do you draw the line?
In the end, how every you implement a check of this nature, you are always going to find people to find work arounds. The trick is going to be balancing the user experience vs the level of enforcement. You can make the check 100% effective by blocking all questions, but that doesn't do anyone any good.
So SE designed the filter to impact the fewest number of users with virtually no chance of false positive as long as the user formats their code as code. Yes it allows questions to sneak through the filter because someone added code tags around a single word, but it also means you won't have a frustrated users screaming at the computer "But I did include code!". And for questions that do sneak through, that is what human moderation is for.
I'd say that if you wanted to make it stricter, then I'd only think about a minimum length longer than a single word. The issue here comes to my 2nd bullet above.... how much is enough? Is 2 words enough? 3? 12? The longer you make the minimum requirement the more chance you have for a false positive.
Maybe the compromise is not making the filter more strict, but designing a click through warning. Yes it can get ignored, but the idea is to tell the user that their question may be considered unacceptable due to the lack of sufficient code when there is a link to a fiddle. You could make this check much more strict and display a yellow popup with a message along the lines of:
Questions should be self contained. In order help you get the best answers possible, questions with links to jsfiddle.net should be accompanied by a block of code sufficient to demonstrate the problem in the question.
You will still block posts that fail to provide any code, but now for users who only provide a little code you will give them some feedback without blocking them completely. So you will still get false positives, but at least the users will still be able to post their question after getting warned. And of course not everyone will read the message but even if 10% of the people read it and 5% actually do something to address the problem, it makes the human side of the moderation easier.