If you can show these people would know how to do it, then it must be common enough knowledge that the courts could find someone else.
If it is highly specialized knowledge that only these people might even know, then you can't prove that they do know how to do it.
Either way, you can't justify forcing them to do it. And it is if we are even fine with slavery to begin with, which I'm not.
[1] I mean this in the sense of to deal with or correct a problem, and also to influence the result by improper or illegal methods
Creating code is not like being compelled to testify. Is the DoJ going to have a prosecutor ask them "Now what would be the next line of code?" until they get a working tool?
The coders have an excellent case against them being jailed. The government has already stated their fallback position - that they want Apple's source code and signing keys. Apple will probably at that point secure their signing keys and code, perhaps by moving them offshore.
It will be interesting to see if some defense contractor can make head or tail of Apple's code, build system, etc. That could be a comedy I'd pay to watch.
If a judge orders an injunction against these citizens and they don't comply, he won't fine them, he will jail them, which is an incredible hardship, esp. against people not used to the prison industrial complex and its dehumanizing effects.
Perhaps sitting in jail for weeks or months will endure these guys to the IT masses as martyrs-to-the-cause, but I would fully expect them to cave after a little while.