This whole approach of copy-pasting an official addon, tweaking it, and releasing it as a third-party addon, is disastrous for compatibility. It's been a perpetual source of compatibility problems, even though we've long asked developers not to do it. The compatibility problems have even involved corrupting user's projects who then panic and we end up supporting them and recovering their projects for them. So we are deliberately hiding the source code to try to prevent this entirely.
Generally there's no need for third-party addons to carry around a copy of a whole official addon with them just to add one small feature, and it adds a large surface area for causing more compatibility problems down the line. We'd rather these plugins add the minimal extra functionality alone without copying any aspect of any other plugin. This makes addons smaller, easier to maintain, far reduces the compatibility headaches that users face, and reduces the burden on our support.
If any plugin developers have questions about how to achieve certain things in the SDK, I'm happy to offer help on the forum, or create additional minimal SDK samples to demonstrate things like attaching to DOM events and such.