Iirc it was implemented with a pretty seamless API at the undocumented ntdll layer, which let you set a current transaction on a handle then do API calls as usual.
The issue is that while NT mostly reasons about files as handles, Win32 deals primarily with the name for many operations. So they duplicated every api that took a filename.
Doesn't surprise me to learn that the NT-level API is nicer.
The NT API is in general much better thought out than the Win32 API. It is a real pity that Microsoft chose to leave the former largely undocumented and tell people to use the later instead.
The issue is that while NT mostly reasons about files as handles, Win32 deals primarily with the name for many operations. So they duplicated every api that took a filename.