Earlier quoted context omitted.
The first amendment argument can be made notwithstanding that corporations are not people. Forcing developers to implement and sign code goes against their first amendment rights.
No developers are being forced to do anything. Any Apple employee who feels like it is more than welcome to quit. Otherwise, they're required to do the job their employer asks them to do which, it turns out, might include modifying iOS on a court order.
You are essentially claiming that a person's rights aren't violated because they still have a choice between expressing a concept in code or losing their livelihood. This is analagous to saying that a person's fifth amendment rights are not violated because they have a choice between answering questions and going to jail. This is the precise definition of coercion, which is exactly what the US government is forbidden to do except in very limited circumstances.
Furthermore, their employer is not the government so the ultimatum does not exist without coercion. Their manager is a human being who is forced to give them orders or fire them, which is again a first amendment violation. They get their orders from their manager, who was coerced, etc.