If you read the obituary of the two Ranger Candidates killed during training there are two facts worth noting that are germane to this thread.
The Officer candidate, 2nd Lt. Evan Fitzgibbon, was still assigned to IBOLC. My understanding is that Ranger School is almost a requirement if the 18A 11A aspires to make O-3. Joining the Ranger Regiment as an 18A 11A would require the tab.
Staff Sgt. George Taber, although already a Green Beret, was not Ranger tabbed, which is why he was a Ranger School. There are GB's, even officers, who are not Ranger tabbed. Not many, but they exist. Just as there are some GB's who didn't start their careers in combat arms. One thing is for sure, if you started out in a support branch and join a team, who better make sure you have the Ranger tab.