I guess it wouldn't even exist if OllyDbg x64 would be a (non-alpha) thing. x64dbg provides a number of plugins in order to fill missing features to IDA/Olly: https://github.com/x64dbg/x64dbg/wiki/Plugins
> x64dbg is licensed under GPLv3, which means you can freely distribute and/or modify the source of x64dbg, as long as you share your changes with us.
Should probably read: "... as long as you make genuine offer of providing the source code and changes to those you distribute your version of x64dbg to."
In practice it of course makes sense to upstream changes, but there's nothing in the gpl about that.
Anyway, I guess I would have reworded it somewhat, to make it more obvious that the source is under GPL, but that the project welcomes and encourages upstreaming changes. This opposed to the code being under a modified GPL.