If you're in an offside position it may be ignored if you make no material impact on play. Law 10.1 "...An offside player must not interfere with play. This includes a. playing the ball" Did his offside position have an impact on (interfere with) play? Yes, he put himself in a position to receive a pass to go on and score. As I mentioned, he would not have been able to receive that pass in that position had he retreated or waited to be put onside. The "moving towards the ball" applies to a player who is offside when a team mate kicks ahead.
It's highly likely that I'm being thick but I don't understand your point about attacking sides moving forward running optimistic lines from ruck to ruck, are you saying players are doing that in front of the ball carrier? The only time you see players moving forward in front of the ball is dummy runners when a ball is passed out of a breakdown position and if they happen to block an opponent who might have made a tackle (i.e. they interfere with play) they will certainly be penalised.
He was onside when he played the ball. Simmonds had overtaken him.
https://www.world.rugby/the-game/laws/law/10There's the laws. I agree that he's offside because he's in front of the carrier. He does not have an impact on play whilst he's in front of the carrier, he only impacts it once he's behind the carrier.
He's not allowed to run forwards towards the ball. The laws don't clarify if that's for kicks or not, either way, it doesn't apply to him, as he's not running towards the ball. That's 10.4b.
The next section does explicitly mention a kicked ball, when it talks about retiring immediately. But the ball wasn't kicked, so he doesn't need to.
As for my point about optimistic lines.
An optimistic scrum half is one that see's the upcoming play after he's passed to his 10 (thus the 9 is now offside, as he's in front of the 10), and he will run forwards and across, staying in front of the play for the majority of it, waiting/expecting a break, when he'll align with the ruck.
You can see Robson doing it a lot, you can see the ABs scrum halves doing it most the time. It saves time and effort on their part, and it also means they're in a better position to support a break. However, they are offside, but as long as they don't interfere with play until onside again, they're fine.