The written agreement was for absolutely NO contact between the cars during racing and respectful distances in the first 5 laps.
Lewis got the better start, got the inside line and drove the racing line. This meant that it was Nico, not Lewis who was forced to agree to the agreement. He had to back out or collide. This was identical to Spa in 14, but in that instance Nico allowed his car to hit Lewis rather than back off. This time, post agreement, he had no choice but to back-off.
If he’d have realised this just a f reaction of a secon earlier, he’d have been able to slot in behind Lewis and ride the sister cars slipstream, rather than holding the challenge to the last possible moment and thn being forced on to the grass, losing 2 positions in the process.