by "illegal", do you mean the State will put you in jail for it? definitionally, yes, in this case.
but now we're allowing that they didn't "just write code", I notice. which was my original point.
Discussion
Why are you trying to distinguish that point? Samourai Wallet did a lot more than just write code, they hosted a node, they hosted a coordinator, they hosted a blockchain analysis platform, they published several research articles, they published several instructional videos, they gave presentations at conferences, they donated $50k to Tor, they were the executive producer of the 3D printed gun documentary Death Athletic, and they ran a social media account. They did quite a lot.
The only think they are going to prison for however, is because they knew a criminal /could/ use their code. That's it. They are going to prison because of that code. So what is this point you're grasping at straws to make?