It's stolen closed source. That doesn't make it open.
In terms of declaring it open source and specifically using it as a development resource for public future development, sticking it on a github, etc, you are correct.
However, to childishly stomp your foot and scream "IST VERBOTEN" ignores the fact that the source text used to describe the final object code it's now effectively everybody's, as a point of reference. Not as a true piece of open source development, but as a discarded scrap of abandonware.
If you want to be ignorant about the current situation, that is your choice, and your choice will be respected, by me, at least.
It is easily available and will remain so, that makes it open.
Disagree. Open source as a development root has a specific definition. Source for 3.1 is now a point of reference, a leaked document in the public domain. No more than that, it is too much of a jump to declare it as a true open source OS. It isn't.
Why? Copyright rests with the copyright holder until 70 years (probably longer) after expiration of the copyright holder.
It becomes Open Source 70 years after CBM filed for Chapter 11.
If in the meantime, it gets wholesale conversion to native HEX code, as opposed to being compiled C, then the translated version could be declared as Open source (translated works are not a technical breach of copyright, although they could be used as the basis for an IP infringement legal case - extremely unlikely). However, such a translation would have the same Amiga problem - it would require a custom ROM version for each basic Amiga type. NOT a small task.