So people who wants to use TMS will need an update where the term "Template Modifications" is changed.
Unlike TMS, XenForo's implementation doesn't show the original content of a template when entering in the textbox:
This means that you need to open the specified template in a new tab and copy&pasta things from there. Auto-completing of template names isn't there neither.
The next thing is the modification key. For each modification you need to specify a modification key. It won't let you save a template modification without entering a modification key. This key is not visible on the template modifications overview page, but on the edit page of template modifications. This shows that this system is really aimed for add-on developers.
Template Comparison works as shown in the Have You Seen section.
What does this mean for Style developers? Obviously style developers can use this system to make styles, but the more convenient way remains the TMS add-on. The major problem I see here is that add-on developers most likely will make use of xenforo's TMS version and if you have also TMS add-on installed, your Template Modifications are spread across two systems, which potentially could cause other problems. So I hope that xenforo further develops its version of TMS in favor of style developers.