Yes, most of those are stills from the video clips that were shown in the documentary Bruce Lee: The Man and the Legend (58:30). The unknown skinny guy is seen in short choreographed fight sequences with Wu Ngan, and with Wong Shun-Leung (wearing blue adidas track pants). The very last scene in the segment shows Bruce Lee from behind (in a two-shot) throwing a punch at Wong Shun-Leung (facing camera) and then at the unknown skinny guy. They seem to be practicing face punch reaction shots. All these scenes have the sound effects dubbed in.
The unknown skinny guy is REALLY skinny...lol. He moves in a weird, awkward way, but he's pretty quick.
I wonder if these fight screen tests were ideas Bruce was working on for ETD, or for GOD?