It depends on how one defines it, and based on how I define it, Daniels is a DT.
To me DEs are players that you would want lining up at 4 and 5 technique spot which is on and outside the OT, which Daniels doesn't have the length you would prefer for someone in that role. As a 3-4 "DE" Daniel has mostly asked to play the 2 and 3 technique spot which is on and outside the OG, which is the classic DT role.
DTs are generally more limited to the interior OL, between the OTs, which Daniels is. And as you mentioned, the roster listed the entire current DL as DTs except Wilkerson, Lowry, Looney and Sheehy, which have the length for 4 and 5 technique spot (though I question Looney a little bit, but I haven't gotten to see him play yet with the Packers).
But in the Packers have had trouble filling the DE 4 and 5 technique roles for years... because Raji, Daniels nor Pickett could play there... so they tried 6'4" NT Guion to fill but he wasn't athletic enough, then it went to 6'4" Pennel who did it alright until he got into the doghouse, then to Lowry, and now to Wilkerson.