Pretty odd. I also don’t see why it would be hard rejected, it’s highly configurable and looks good (though admin-side design is not too important for plugins like this). If there was an issue with the code, he would have mentioned the issues in a soft rejection.
There’s some CoinHive alternatives popping up, maybe if you add support for some of them into your plugin and let users choose which API they wish to use, you can resubmit and give it another shot. 