Even though Jin had activated the System Rider belt, he did not fully finish the sequence. Instead, he plunged himself right before the group of police assault squad was still performing their fire suppression duty to keep the Tri-Rexes at bay. Unlike the Tri-Rex who was being ridiculed at a variety of angles and had trouble keeping killing considering the amount of soldiers that were against it, the Tri-Rexes on the 35th floor felt that those bullets were like raindrops from a mild sunny shower.
They were basking in that 'rain' and were not eager to retaliate back as if to mock the soldiers who were firing desperately at them in the efforts to stop them from coming up. That was when Jin could have deduced that the Tri-Rex Mechanoids were not communicating with each other and rather it was kill anything on sight. Commands were not given properly seeing how the Tri-Rex killed their own gorilla Mechanoid.
