One pulse bounces between A and B. Every photon — the pulse and everything it scatters — moves at the same fixed speed (30 units/s, 40 scatter points, actual one-way time 3.33 s). Both columns are the same event, filmed from two places, perfectly in sync.
Same pulse, same instant — but C₁ (equidistant from both ends) films a symmetric round trip, while C₂ (beside A) films the A→B leg slow (each scatter point's report comes from farther away, stretching the leg to about twice its real time) and the B→A leg fast (the pulse chases its own scattered light home, so the whole return arrives almost at once).
No relativity involved — every photon here moves at one constant speed. The camera image is idealized: each flash is drawn at the true position along A–B it came from, at the moment its scattered photon reaches the camera.