Developers say they dont want to mess up with application hotkeys which is reasonable. Some applications already use CTRl+T for something.
To achieve your purpose just use WIN+E (and auto group explorer).
The reason why other software supports CTRL+T is that they only embedded windows explorer while groupy works with all applications. Perhaps developers of groupy will make a "Windows explorer" only group which then supports these hotkeys. I believe that many people are only interested into grouping windows explorer (such as myself).
Another solution would be adding a plus button which launches an additional instance of the current selected tab. I think Microsoft's new UWP group thing does it this way.
In the end there are multiple solutions, none is 100% perfect but any of them is better than it is right now.