It was originally, right? Because if it was the Tesselecta all along, then River Song not shooting wouldn't have caused time to fracture. The Doctor changed his future by putting the Tesselecta there in Wedding. Anyhow, if I'm remembering correctly River Song explains in TIA that the "spaceman" had to shoot him twice to prevent regeneration. It wouldn't need preventing if he couldn't do it (though did River know about 8.5?).