Yes, it's Foul Play, but there are different levels of Foul Play, ranging from technical offences up to the worst.
I haven't read the "simplified" lawbook but I'm sure there was a section in the previous book that said an offence by the scoring side could effectively be ignored if the try would probably have been scored anyway.
This was not a technical offence. It was tackling or obstructing a player without the ball. What do you usually do in such circumstances in open play? Simply ignore it?