Sigil Captain's ability checks a creature's initial power and toughness upon being put on the battlefield, so it will take into account counters that it enters with and static abilities that may give it a continuous power boost once it's on the battlefield (such as the one on Glorious Anthem). After the creature is already on the battlefield, changing its power and toughness to 1/1 with a spell, activated ability, or triggered ability won't allow this ability to trigger; it's too late by then.