If you had watched the video you would have seen that the player in question was near the point where the run ended. He had to cross the field to join the opposing huddle near the sideline.
You are observant and correct, however White #3 doesn't appear to have done anything illegal. Absent any previous problems with excess tension between teams, or with the Blue coaching staff, an official simply injecting himself between White #3 and the Blue huddle, and separating them either by directing White # 3 to move away from the huddle, or moving the Blue huddle into the team area would have avoided what, on the surface, and in the context of this video alone, seems like an avoidable flag.