You just found the issue. We have not officially claimed support DFL/FFL for 2016.
I need to report this as a bug if a 2016 DFL looks to us as 2003.
Force the join and move ahead, report any other issues.
Tip: I am not a DM exptert, but you could modify the LUA file that does the adcheck not to do the AD check so you can carry on with it.
R.P