I don't think it said anything at all to be honest.
At the time Harry was in front of Johnson he looked at where Megan was and couldn't help but see Wills and Katherine were on their way to entering Westminster and joining the greeting line up. He then started to move forward pretty quickly. Megan spoke to Johnson a while then caught sight of Wills and Kate and she too moved forward quickly as it was simply the polite thing to do. I am sure they did not want any suggestion they were hogging the limelight so to speak.
It's like the BBC commentator, I think it was Simon McCoy, commenting when Charles and Johnson were talking and Charles said something to Johnson and he raised his left hand with a pointed finger in a gesture which implied ' you' ve got it'. McCoy said ' woops, the PM went to shake hands', No he didn't.
The private meeting a few weeks ago between Johnson and Harry was said to be convivial so I am sure Harry would not purposely be rude to the PM, it's not his style.