Construct's code is not open source.
You cannot redistribute Construct's code under an open source license, because it is not your code and you don't have permission to do that.
You can open source your Construct project file, though. This is what "open source" means to most people working on a Construct project.
That is the end of the story. There is no way around this. There is no point trying to argue around this using different formats or export options. Construct's code is not open source and cannot be distributed as such.